Please refer to the errata for this document, which may include some normative corrections.
See also translations.
Copyright © 2009 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
This document defines the Simple Knowledge Organization System (SKOS), a common data model for sharing and linking knowledge organization systems via the Web.
Many knowledge organization systems, such as thesauri, taxonomies, classification schemes and subject heading systems, share a similar structure, and are used in similar applications. SKOS captures much of this similarity and makes it explicit, to enable data and technology sharing across diverse applications.
The SKOS data model provides a standard, low-cost migration path for porting existing knowledge organization systems to the Semantic Web. SKOS also provides a lightweight, intuitive language for developing and sharing new knowledge organization systems. It may be used on its own, or in combination with formal knowledge representation languages such as the Web Ontology language (OWL).
This document is the normative specification of the Simple Knowledge Organization System. It is intended for readers who are involved in the design and implementation of information systems, and who already have a good understanding of Semantic Web technology, especially RDF and OWL.
For an informative guide to using SKOS, see the [SKOS-PRIMER].
Using SKOS, concepts can be identified using URIs, labeled with lexical strings in one or more natural languages, assigned notations (lexical codes), documented with various types of note, linked to other concepts and organized into informal hierarchies and association networks, aggregated into concept schemes, grouped into labeled and/or ordered collections, and mapped to concepts in other schemes.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This document is a W3C Recommendation developed by the Semantic Web Deployment Working Group, part of the W3C Semantic Web Activity. This document reflects an editorial change arising during the Proposed Recommendation review: a non-normative example and preceding text was removed that suggested one means to reference a system of notation (e.g. a symbolic notation) in a label where the system of notation does not correspond to a natural language. This suggestion was deemed inconsistent with IETF Best Current Practice 47 on the use of tags for identifying languages. Users should consider the SKOS Extension vocabulary for support of alternate systems of notation. An implementation report documents known uses of SKOS during the Candidate Recommendation review period. An updated SKOS Primer is being published concurrently with this Recommendation.
Changes Since 15 June 2009 Proposed Recommendation:
Comments on this document may be sent to<EMAIL_ADDRESS>with public archive.
This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.
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Classes
Properties
Sections
Appendices
The Simple Knowledge Organization System is a data-sharing standard, bridging several different fields of knowledge, technology and practice.
In the library and information sciences, a long and distinguished heritage is devoted to developing tools for organizing large collections of objects such as books or museum artifacts. These tools are known generally as "knowledge organization systems" (KOS) or sometimes as "controlled structured vocabularies". Several similar yet distinct traditions have emerged over time, each supported by a community of practice and set of agreed standards. Different families of knowledge organization systems, including thesauri, classification schemes, subject heading systems, and taxonomies are widely recognized and applied in both modern and traditional information systems. In practice it can be hard to draw an absolute distinction between thesauri and classification schemes or taxonomies, although some properties can be used to broadly characterize these different families (see e.g., [BS8723-3]). The important point for SKOS is that, in addition to their unique features, each of these families shares much in common, and can often be used in similar ways [SKOS-UCR]. However, there is currently no widely deployed standard for representing these knowledge organization systems as data and exchanging them between computer systems.
The W3C's Semantic Web Activity [SW] has stimulated a new field of integrative research and technology development, at the boundaries between database systems, formal logic and the World Wide Web. This work has led to the development of foundational standards for the Semantic Web. The Resource Description Framework (RDF) provides a common data abstraction and syntax for the Web [RDF-PRIMER]. The RDF Vocabulary Description language (RDFS) and the Web Ontology language (OWL) together provide a common data modeling (schema) language for data in the Web [RDFS] [OWL-GUIDE]. The SPARQL Query Language and Protocol provide a standard means for interacting with data in the Web [SPARQL].
These technologies are being applied across diverse applications, because many applications require a common framework for publishing, sharing, exchanging and integrating ("joining up") data from different sources. The ability to link data from different sources is motivating many projects, as different communities seek to exploit the hidden value in data previously spread across isolated sources.
One facet of the Semantic Web vision is the hope of better organizing the vast amounts of unstructured (i.e., human-readable) information in the Web, providing new routes to discovering and sharing that information. RDFS and OWL are formally defined knowledge representation languages, providing ways of expressing meaning that are amenable to computation, meaning that complements and gives structure to information already present in the Web [RDF-PRIMER] [OWL-GUIDE]. However, to actually apply these technologies over large bodies of information requires the construction of detailed maps of particular domains of knowledge, in addition to the accurate description (i.e., annotation or cataloging) of information resources on a large scale, much of which cannot be done automatically. The accumulated experience and best practices in the library and information sciences regarding the organization of information and knowledge are obviously complementary and applicable to this vision, as are the many existing knowledge organization systems already developed and in use, such as the Library of Congress Subject Headings [LCSH] or the United Nations Food and Agriculture Organization's AGROVOC Thesaurus [AGROVOC].
The Simple Knowledge Organization System therefore aims to provide a bridge between different communities of practice within the library and information sciences involved in the design and application of knowledge organization systems. In addition, SKOS aims to provide a bridge between these communities and the Semantic Web, by transferring existing models of knowledge organization to the Semantic Web technology context, and by providing a low-cost migration path for porting existing knowledge organization systems to RDF.
Looking to the future, SKOS occupies a position between the exploitation and analysis of unstructured information, the informal and socially-mediated organization of information on a large scale, and the formal representation of knowledge. By making the accumulated experience and wisdom of knowledge organization in the library and information sciences accessible, applicable and transferable to the technological context of the Semantic Web, in a way that is complementary to existing Semantic Web technology (and in particular formal systems of knowledge representation such as OWL), it is hoped that SKOS will enable many new and valuable applications, and will also lead to new integrative lines of research and development in both technology and practice.
The Simple Knowledge Organization System is a common data model for knowledge organization systems such as thesauri, classification schemes, subject heading systems and taxonomies. Using SKOS, a knowledge organization system can be expressed as machine-readable data. It can then be exchanged between computer applications and published in a machine-readable format in the Web.
The SKOS data model is formally defined in this specification as an OWL Full ontology [OWL-SEMANTICS]. SKOS data are expressed as RDF triples [RDF-CONCEPTS], and may be encoded using any concrete RDF syntax (such as RDF/XML [RDF-XML] or Turtle [TURTLE]). For more on the relationships between SKOS, RDF and OWL, see the next sub-section below.
The SKOS data model views a knowledge organization system as a concept scheme comprising a set of concepts. These SKOS concept schemes and SKOS concepts are identified by URIs, enabling anyone to refer to them unambiguously from any context, and making them a part of the World Wide Web. See Section 3. The skos:Concept Class for more on identifying and describing SKOS concepts, and Section 4. Concept Schemes for more on concept schemes.
SKOS concepts can be labeled with any number of lexical (UNICODE) strings, such as "romantic love" or "れんあい", in any given natural language, such as English or Japanese (written here in hiragana). One of these labels in any given language can be indicated as the preferred label for that language, and the others as alternative labels. Labels may also be "hidden", which is useful where a knowledge organization system is being queried via a text index. See Section 5. Lexical Labels for more on the SKOS lexical labeling properties.
SKOS concepts can be assigned one or more notations, which are lexical codes used to uniquely identify the concept within the scope of a given concept scheme. While URIs are the preferred means of identifying SKOS concepts within computer systems, notations provide a bridge to other systems of identification already in use such as classification codes used in library catalogs. See Section 6. Notations for more on notations.
SKOS concepts can be documented with notes of various types. The SKOS data model provides a basic set of documentation properties, supporting scope notes, definitions and editorial notes, among others. This set is not meant to be exhaustive, but rather to provide a framework that can be extended by third parties to provide support for more specific types of note. See Section 7. Documentation Properties for more on notes.
SKOS concepts can be linked to other SKOS concepts via semantic relation properties. The SKOS data model provides support for hierarchical and associative links between SKOS concepts. Again, as with any part of the SKOS data model, these can be extended by third parties to provide support for more specific needs. See Section 8. Semantic Relations for more on linking SKOS concepts.
SKOS concepts can be grouped into collections, which can be labeled and/or ordered. This feature of the SKOS data model is intended to provide support for node labels within thesauri, and for situations where the ordering of a set of concepts is meaningful or provides some useful information. See Section 9. Concept Collections for more on collections.
SKOS concepts can be mapped to other SKOS concepts in different concept schemes. The SKOS data model provides support for four basic types of mapping link: hierarchical, associative, close equivalent and exact equivalent. See Section 10. Mapping Properties for more on mapping.
Finally, an optional extension to SKOS is defined in Appendix B. SKOS eXtension for Labels (SKOS-XL). SKOS-XL provides more support for identifying, describing and linking lexical entities.
The elements of the SKOS data model are classes and properties, and the structure and integrity of the data model is defined by the logical characteristics of, and interdependencies between, those classes and properties. This is perhaps one of the most powerful and yet potentially confusing aspects of SKOS, because SKOS can, in more advanced applications, also be used side-by-side with OWL to express and exchange knowledge about a domain. However, SKOS is not a formal knowledge representation language.
To understand this distinction, consider that the "knowledge" made explicit in a formal ontology is expressed as sets of axioms and facts. A thesaurus or classification scheme is of a completely different nature, and does not assert any axioms or facts. Rather, a thesaurus or classification scheme identifies and describes, through natural language and other informal means, a set of distinct ideas or meanings, which are sometimes conveniently referred to as "concepts". These "concepts" may also be arranged and organized into various structures, most commonly hierarchies and association networks. These structures, however, do not have any formal semantics, and cannot be reliably interpreted as either formal axioms or facts about the world. Indeed they were never intended to be so, for they serve only to provide a convenient and intuitive map of some subject domain, which can then be used as an aid to organizing and finding objects, such as documents, which are relevant to that domain.
To make the "knowledge" embedded in a thesaurus or classification scheme explicit in any formal sense requires that the thesaurus or classification scheme be re-engineered as a formal ontology. In other words, some person has to do the work of transforming the structure and intellectual content of a thesaurus or classification scheme into a set of formal axioms and facts. This work of transformation is both intellectually demanding and time consuming, and therefore costly. Much can be gained from using thesauri, etc., as-is, as informal, convenient structures for navigation within a subject domain. Using them as-is does not require any re-engineering and is therefore much less costly. In addition, some KOS are, by design, not intended to represent a logical view of their domain. Converting such KOS to a formal logic-based representation may, in practice, involve changes which result in a representation that no longer meets the originally intended purpose.
OWL does, however, provide a powerful data modeling language. We can, therefore, use OWL to construct a data model for representing thesauri or classification schemes as-is. This is exactly what SKOS does. Taking this approach, the "concepts" of a thesaurus or classification scheme are modeled as individuals in the SKOS data model, and the informal descriptions about and links between those "concepts" as given by the thesaurus or classification scheme are modeled as facts about those individuals, never as class or property axioms. Note that these are facts about the thesaurus or classification scheme itself, such as "concept X has preferred label 'Y' and is part of thesaurus Z"; these are not facts about the way the world is arranged within a particular subject domain, as might be expressed in a formal ontology.
SKOS data are then expressed as RDF triples. The RDF graph below (in [TURTLE] as discussed in Section 1.7.3) expresses some facts about a thesaurus.
<A> rdf:type skos:Concept ; skos:prefLabel "love"@en ; skos:altLabel "adoration"@en ; skos:broader <B> ; skos:inScheme <S> . <B> rdf:type skos:Concept ; skos:prefLabel "emotion"@en ; skos:altLabel "feeling"@en ; skos:topConceptOf <S> . <S> rdf:type skos:ConceptScheme ; dct:title "My First Thesaurus" ; skos:hasTopConcept <B> .
This point is vital to understanding the formal definition of the SKOS data model and how it may be implemented in software systems. This point is also vital to more advanced applications of SKOS, especially where SKOS and OWL are used in combination as part of a hybrid formal/semi-formal design.
From a user's point of view, however, the distinction between a formal
knowledge representation system and an informal or semi-formal knowledge
organization system may naturally become blurred. In other words, it may not
be relevant to a user that <A>
and <B>
in the graph below are individuals (instances of skos:Concept
),
and <C>
and <D>
are classes (instances
of owl:Class
) .
<A> rdf:type skos:Concept ; skos:prefLabel "love"@en ; skos:broader <B> . <B> rdf:type skos:Concept ; skos:prefLabel "emotion"@en . <C> rdf:type owl:Class ; rdfs:label "mammals"@en ; rdfs:subClassOf <D> . <D> rdf:type owl:Class ; rdfs:label "animals"@en .
An information system that has any awareness of the SKOS data model will, however, need to appreciate the distinction.
RDF schemas for SKOS and the SKOS eXtension for Labels (SKOS-XL) are described in Appendix C. SKOS and SKOS-XL Namespace Documents. Note that, as there are constraints that cannot be completely captured in the schema, the RDF/XML document provides a normative subset of this specification.
Under the RDF and OWL Full semantics, the formal meaning (interpretation) of an RDF graph is a truth value [RDF-SEMANTICS] [OWL-SEMANTICS], i.e., an RDF graph is interpreted as either true or false.
In general, an RDF graph is said to be inconsistent if it cannot possibly be true. In other words, an RDF graph is inconsistent if it contains a contradiction.
Using the RDF and RDFS vocabularies alone, it is virtually impossible to make a contradictory statement. When the OWL vocabulary is used as well, there are many ways to state a contradiction, e.g., consider the RDF graph below.
<Dog> rdf:type owl:Class . <Cat> rdf:type owl:Class . <Dog> owl:disjointWith <Cat> . <dogcat> rdf:type <Dog> , <Cat> .
The graph states that <Dog>
and
<Cat>
are both classes, and that they are disjoint, i.e.,
that they do not have any members in common. This is contradicted by the
statement that <dogcat>
has type both
<Dog>
and <Cat>
. There is no OWL Full
interpretation which can satisfy this graph, and therefore this graph is
not OWL Full consistent.
When OWL Full is used as a knowledge representation language, the notion of inconsistency is useful because it reveals contradictions within the axioms and facts that are asserted in an ontology. By resolving these inconsistencies we learn more about a domain of knowledge, and come to a better model of that domain from which interesting and valid inferences can be drawn.
When OWL Full is used as a data modeling (i.e., schema) language, the notion of inconsistency is again useful, but in a different way. Here we are not concerned with the logical consistency of human knowledge itself. We are simply interested in formally defining a data model, so that we can establish with certainty whether or not some given data fit with (i.e., conform to) the given data model. If the data are inconsistent with respect to the data model, then the data does not fit.
Here, we are not concerned with whether or not some given data have any correspondence with the real world, i.e., whether they are true or false in any absolute sense. We are simply interested in whether or not the data fit the data model, because interoperability within a given class of applications depends on data conforming to a common data model.
Another way to express this view is via the notion of integrity.
Integrity conditions are statements within the formal definition of a data
model, which are used to establish whether or not given data are consistent
with respect to the data model, e.g., the statement that <Dog>
and
<Cat>
are disjoint classes can be viewed as an integrity
condition on a data model. Given this condition, the data below are then not consistent.
<dogcat> rdf:type <Dog> , <Cat> .
The definition of the SKOS data model given in this document contains a limited number of statements that are intended as integrity conditions. These integrity conditions are included to promote interoperability, by defining the circumstances under which data are not consistent with respect to the SKOS data model. Tools can then be implemented which check whether some or all of these integrity conditions are met for given data, and therefore whether the data are consistent with the SKOS data model.
These integrity conditions are part of the formal definition of the classes and properties of the SKOS data model. However, they are presented separately from other parts of the formal definition because they serve a different purpose. Integrity conditions serve primarily to establish whether given data are consistent with the SKOS data model. All other statements within the definition of the SKOS data model serve only to support logical inferences. (See also the next sub-section.)
Integrity conditions are defined for the SKOS data model in a way that is independent of strategies for their implementation, in so far as that is possible. This is because there are several different ways in which a procedure to find inconsistencies with the SKOS data model could be implemented. Inconsistencies could be found using an OWL reasoner. Alternatively, some inconsistencies could be found by searching for specific patterns within the data, or by a hybrid strategy (e.g., drawing inferences using an RDFS or OWL reasoner, then searching for patterns in the inferred graph).
The integrity conditions on the SKOS data model are fewer than might be expected, especially for those used to working within the closed world of database systems. See also the next sub-section, and the notes in sections 3-10 below.
This document defines the SKOS data model as an OWL Full ontology. There are other ways in which the SKOS data model could have been defined, for example as an entity-relationship model, or a UML class model. Although OWL Full as a data modeling language appears intuitively similar in many ways to these other modeling approaches, there is an important fundamental distinction.
RDF and OWL Full are designed for systems in which data may be widely distributed (e.g., the Web). As such a system becomes larger, it becomes both impractical and virtually impossible to know where all of the data in the system are located. Therefore, one cannot generally assume that data obtained from such a system are complete. If some data appear to be missing, one has to assume, in general, that the data might exist somewhere else in the system. This assumption, roughly speaking, is known as the open world assumption [OWL-GUIDE].
This means in practice that, for a data model defined as an OWL Full
ontology, some definitions can have a counter-intuitive meaning. No
conclusions can be drawn from missing data, and removing something will
never make the remaining data inconsistent. This is illustrated, for example, by the definition of skos:semanticRelation
in Section 8 below. The
property skos:semanticRelation
is defined to have domain and
range skos:Concept
. These domain and range definitions give
license to inferences. Consider the graph below.
<A> skos:semanticRelation <B>.
In this case, the graph above entails the following graph.
<A> rdf:type skos:Concept . <B> rdf:type skos:Concept .
Thus, we do not need to explicitly state here that
<A>
and <B>
are instances of
skos:Concept
, because such statements are entailed by the definition of skos:semanticRelation
.
In the SKOS data model, most statements of definition are
not integrity conditions, but are statements of logical
dependency between different elements of the data model, which (under the
open world assumption) give license to a number of simple inferences. This is illustrated, for
example, by the statement in Section 7 below that
skos:broader
and skos:narrower
are
inverse properties. This statement means that
<A> skos:narrower <B> .
entails
<B> skos:broader <A> .
Both of these two graphs are, by themselves, consistent with the SKOS data model.
Knowledge organization systems such as thesauri and classification schemes are applied in a wide range of situations, and an individual knowledge organization system can be used in many different information systems. By defining the SKOS data model as an OWL Full ontology, the Semantic Web can then be used as a medium for publishing, exchanging, sharing and linking data involving these knowledge organization systems. For this reason, for the expressiveness of OWL Full as a data modeling language, and for the possibility of using thesauri, classification schemes, etc., side-by-side with formal ontologies, OWL Full has been used to define the SKOS data model. The open world assumption is therefore a fundamental premise of the SKOS data model, and should be borne in mind when reading this document.
See also [RDF-PRIMER] and [OWL-GUIDE].
skos:hasTopConcept
), the constraint has not been stated
formally. In some cases, although no formal constraint is stated, usage conventions
are recommended. Applications that require more constrained behaviour may define
extensions to the SKOS vocabulary. See also the [SKOS-PRIMER].
This document formally defines the Simple Knowledge Organization System data model as an OWL Full ontology. The elements of the SKOS data model are OWL classes and properties, and a Uniform Resource Identifier (URI) is provided for each of these classes and properties so that they may be used unambiguously in the Web. This set of URIs is the SKOS vocabulary.
The complete SKOS vocabulary is given in section 2 below. Sections 3 to 10 then formally define the SKOS data model. The definition of the data model is broken down into a number of sections purely for convenience. Each of these sections 3 to 10 follows a common layout:
Most of the class and property definitions and integrity conditions stated in this document could be stated as RDF triples, using the RDF, RDFS and OWL vocabularies. However, a small number cannot, either because of limitations in the expressiveness of OWL Full or lack of a standard URI for some class. To improve the overall readability of this document, rather than mix RDF triples and other notations, the formal definitions and integrity conditions are stated throughout using prose.
The style of this prose generally follows the style used in [RDFS], and should be clear to a reader with a working knowledge of RDF and OWL.
So, for example, "ex:Person
is an instance of
owl:Class
" means
ex:Person rdf:type owl:Class .
"ex:hasParent
and ex:hasMother
are each
instances of owl:ObjectProperty
" means:
ex:hasParent rdf:type owl:ObjectProperty . ex:hasMother rdf:type owl:ObjectProperty .
"ex:hasMother
is a sub-property of ex:hasParent
"
means
ex:hasMother rdfs:subPropertyOf ex:hasParent .
"the rdfs:range
of ex:hasParent
is the class
ex:Person
" means:
ex:hasParent rdfs:range ex:Person .
Where some formal aspects of the SKOS data model cannot be stated as RDF
triples using either RDF, RDFS or OWL vocabularies, it should be clear to a
reader with a basic understanding of the RDF and OWL semantics how these
statements might be translated into formal conditions on the interpretation
of an RDF vocabulary (e.g., from Section 5, "A resource has no more
than one value of skos:prefLabel
per language tag" means for any
resource x, no two members of the set { y | <x,y> is in
IEXT(I(skos:prefLabel
)) } share the same language tag, where I
and IEXT are functions as defined in [RDF-SEMANTICS]).
Full URIs are cited in the text of this document in monospace font,
enclosed by angle brackets, e.g.,
<http://example.org/ns/example>
. Relative URIs are cited
in the same way, and are relative to the base URI
<http://example.org/ns/>
, e.g.,
<example>
and
<http://example.org/ns/example>
are the same URI.
URIs are also cited in the text of this document in an abbreviated form. Abbreviated URIs are cited in monospace font without angle brackets, and should be expanded using the table of abbreviations below.
| URI | Abbreviation |
|---|---|
| http://www.w3.org/2004/02/skos/core# | skos: |
| http://www.w3.org/1999/02/22-rdf-syntax-ns# | rdf: |
| http://www.w3.org/2000/01/rdf-schema# | rdfs: |
| http://www.w3.org/2002/07/owl# | owl: |
So, for example, skos:Concept
is an abbreviation of
<http://www.w3.org/2004/02/skos/core#Concept>
.
Examples of RDF graphs are given using the Terse RDF Triple language (Turtle) [TURTLE]. All examples assume that they are preceded by the following prefix and URI base directives:
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@base <http://example.org/ns/> .
Therefore, the example given below
| Example 1 |
|---|
|
<MyConcept> rdf:type skos:Concept .
|
is equivalent to the following Turtle document
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> . @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> . @prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix skos: <http://www.w3.org/2004/02/skos/core#> .
@base <http://example.org/ns/> . <MyConcept> rdf:type skos:Concept .
which is equivalent to the following RDF/XML document [RDF-XML]
<?xml version="1.0" encoding="UTF-8"?> <rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xmlns:skos="http://www.w3.org/2004/02/skos/core#"
xmlns:owl="http://www.w3.org/2002/07/owl#"
xml:base="http://example.org/ns/"> <skos:Concept rdf:about="MyConcept"/> </rdf:RDF>
which is equivalent to the following N-TRIPLES document [NTRIPLES]
<http://example.org/ns/MyConcept> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2004/02/skos/core#Concept> .
Note the use in Turtle of the ";" and "," characters to abbreviate multiple triples with the same subject or predicate. Some examples also make use of the Turtle syntax "(...)", representing an RDF Collection.
This specification does not define a formal notion of conformance.
However, an RDF graph will be inconsistent with the SKOS data model if that graph and the SKOS data model (as defined formally below) taken together lead to a logical contradiction.
Where URIs are used to identify resources of type
skos:Concept
, skos:ConceptScheme
,
skos:Collection
or skosxl:Label
, this specification
does not require specific behavior when dereferencing those
URIs via the Web [WEBARCH]. It is, however, strongly recommended that
publishers of SKOS data follow the guidelines given in [COOLURIS] and [RECIPES].
The SKOS namespace URI is:
The SKOS vocabulary is a set of URIs, given in the left-hand column in the table below.
All URIs in the SKOS vocabulary are constructed by appending a local name (e.g., "prefLabel") to the SKOS namespace URI.
See also the SKOS overview in Appendix B and the quick access panel.
The class skos:Concept
is the class of SKOS concepts.
A SKOS concept can be viewed as an idea or notion; a unit of thought. However, what constitutes a unit of thought is subjective, and this definition is meant to be suggestive, rather than restrictive.
The notion of a SKOS concept is useful when describing the conceptual or intellectual structure of a knowledge organization system, and when referring to specific ideas or meanings established within a KOS.
Note that, because SKOS is designed to be a vehicle for representing semi-formal KOS, such as thesauri and classification schemes, a certain amount of flexibility has been built in to the formal definition of this class.
See the [SKOS-PRIMER] for more examples of identifying and describing SKOS concepts.
skos:Concept |
| S1 | skos:Concept is an instance of
owl:Class . |
The graph below states that <MyConcept>
is a SKOS
concept (i.e., an instance of skos:Concept
).
| Example 2 (consistent) |
|---|
|
<MyConcept> rdf:type skos:Concept .
|
Other than the assertion that skos:Concept
is an instance of
owl:Class
, this specification does not make any
additional statement about the formal relationship between the class of SKOS
concepts and the class of OWL classes. The decision not to
make any such statement has been made to allow applications the freedom to
explore different design patterns for working with SKOS in combination with
OWL.
In the example graph below, <MyConcept>
is an
instance of skos:Concept
and an instance of
owl:Class
.
| Example 3 (consistent) |
|---|
|
<MyConcept> rdf:type skos:Concept , owl:Class .
|
This example is consistent with the SKOS data model.
Similarly, this specification does not make any statement about the formal relationship between the class of SKOS concepts and the class of OWL properties.
In the example graph below, <MyConcept>
is
an instance of skos:Concept
and an
instance of
owl:ObjectProperty
.
| Example 4 (consistent) |
|---|
|
<MyConcept> rdf:type skos:Concept , owl:ObjectProperty .
|
This example is consistent with the SKOS data model.
A SKOS concept scheme can be viewed as an aggregation of one or more SKOS concepts. Semantic relationships (links) between those concepts may also be viewed as part of a concept scheme. This definition is, however, meant to be suggestive rather than restrictive, and there is some flexibility in the formal data model stated below.
The notion of a concept scheme is useful when dealing with data from an unknown source, and when dealing with data that describes two or more different knowledge organization systems.
See the [SKOS-PRIMER] for more examples of identifying and describing concept schemes.
skos:ConceptScheme |
skos:inScheme |
skos:hasTopConcept |
skos:topConceptOf |
| S2 | skos:ConceptScheme is an instance of
owl:Class . |
| S3 | skos:inScheme , skos:hasTopConcept and
skos:topConceptOf are each instances of
owl:ObjectProperty . |
| S4 | The rdfs:range of skos:inScheme is the
class skos:ConceptScheme . |
| S5 | The rdfs:domain of skos:hasTopConcept is
the class skos:ConceptScheme . |
| S6 | The rdfs:range of skos:hasTopConcept is
the class skos:Concept . |
| S7 | skos:topConceptOf is a sub-property of
skos:inScheme . |
| S8 | skos:topConceptOf is owl:inverseOf
the property skos:hasTopConcept . |
| S9 | skos:ConceptScheme is disjoint with
skos:Concept . |
The graph below describes a concept scheme with two SKOS concepts, one of which is a top-level concept in that scheme.
| Example 5 (consistent) |
|---|
|
<MyScheme> rdf:type skos:ConceptScheme ;
skos:hasTopConcept <MyConcept> . <MyConcept> skos:topConceptOf <MyScheme> . <AnotherConcept> skos:inScheme <MyScheme> . |
The notion of an individual SKOS concept scheme corresponds roughly to the notion of an individual thesaurus, classification scheme, subject heading system or other knowledge organization system.
However, in most current information systems, a thesaurus or classification scheme is treated as a closed system — conceptual units defined within that system cannot take part in other systems (although they can be mapped to units in other systems).
Although SKOS does take a similar approach, there are no conditions preventing a SKOS concept from taking part in zero, one, or more than one concept scheme.
So, for example, in the graph below the SKOS concept
<MyConcept>
takes part in two different concept schemes
— this is consistent with the SKOS data model.
| Example 6 (consistent) |
|---|
|
<MyScheme> rdf:type skos:ConceptScheme .
<AnotherScheme> rdf:type skos:ConceptScheme ; owl:differentFrom <MyScheme> . <MyConcept> skos:inScheme <MyScheme> , <AnotherScheme> . |
This flexibility is desirable because it allows, for example, new concept schemes to be described by linking two or more existing concept schemes together.
Also, note that there is no way to close the boundary of a concept scheme.
So, while it is possible using skos:inScheme
to say that SKOS
concepts X, Y and Z take part in concept scheme A, there is no way to say
that only X, Y and Z take part in A.
Therefore, while SKOS can be used to describe a concept scheme, SKOS does not provide any mechanism to completely define a concept scheme.
This specification does not make any statement about the formal relationship between the class of SKOS concept schemes and the class of OWL ontologies. The decision not to make any such statement has been made to allow different design patterns to be explored for using SKOS in combination with OWL [OWL-GUIDE].
In the example graph below, <MyScheme>
is
both a SKOS concept scheme and an OWL ontology. This
is consistent with the SKOS data model.
| Example 7 (consistent) |
|---|
|
<MyScheme> rdf:type skos:ConceptScheme , owl:Ontology .
<MyConcept> skos:inScheme <MyScheme> . |
The property skos:hasTopConcept
is, by convention, used to
link a concept scheme to the SKOS concept(s) which are topmost in the
hierarchical relations for that scheme. However, there are no integrity
conditions enforcing this convention. Therefore, the graph below, whilst not
strictly adhering to the usage convention for
skos:hasTopConcept
, is nevertheless consistent
with the SKOS data model.
| Example 8 (consistent) |
|---|
|
<MyScheme> skos:hasTopConcept <MyConcept> .
<MyConcept> skos:broader <AnotherConcept> . <AnotherConcept> skos:inScheme <MyScheme> . |
An application may reject such data but is not required to.
A link between two SKOS concepts does not entail containment within the same concept scheme. This is illustrated in the example below.
| Example 9 (non-entailment) |
|---|
|
<A> skos:narrower <B> .
<A> skos:inScheme <MyScheme> . does not entail
<B> skos:inScheme <MyScheme> .
|
See also Section 8 below.
Note that no domain is stated for the property
skos:inScheme
, i.e., the domain is effectively the class of all
resources (rdfs:Resource
). The decision not to state any domain
has been made to provide some flexibility, enabling extensions to SKOS to
define new classes of resource but still use skos:inScheme
to
link them to a skos:ConceptScheme
. See also example 82 below.
A lexical label is a string of UNICODE characters, such as "romantic love" or "れんあい", in a given natural language, such as English or Japanese (written here in hiragana).
The Simple Knowledge Organization System provides some basic vocabulary for associating lexical labels with resources of any type. In particular, SKOS enables a distinction to be made between the preferred, alternative and "hidden" lexical labels for any given resource.
The preferred and alternative labels are useful when generating or creating human-readable representations of a knowledge organization system. These labels provide the strongest clues as to the meaning of a SKOS concept.
The hidden labels are useful when a user is interacting with a knowledge organization system via a text-based search function. The user may, for example, enter mis-spelled words when trying to find a relevant concept. If the mis-spelled query can be matched against a hidden label, the user will be able to find the relevant concept, but the hidden label won't otherwise be visible to the user (so further mistakes aren't encouraged).
Formally, a lexical label is an RDF plain literal [RDF-CONCEPTS]. An RDF plain literal is composed of a lexical form, which is a string of UNICODE characters, and an optional language tag, which is a string of characters conforming to the syntax defined by [BCP47].
See the [SKOS-PRIMER] for more examples of labeling SKOS concepts. Note especially that the examples below serve only to illustrate general features of the SKOS data model, and do not necessarily indicate best practice for the provision of labels with different language tags. The SKOS Reference aims to establish a data model that is applicable across a range of situations, which may then be refined and/or constrained by usage conventions for more specific situations. Application- and language-specific usage conventions with respect to labels and language tags are out of scope for the SKOS Reference.
skos:prefLabel |
skos:altLabel |
skos:hiddenLabel |
| S10 |
skos:prefLabel ,
skos:altLabel and skos:hiddenLabel are each
instances of owl:AnnotationProperty . |
| S11 |
skos:prefLabel ,
skos:altLabel and skos:hiddenLabel are each
sub-properties of rdfs:label . |
| S12 | The rdfs:range of each of skos:prefLabel ,
skos:altLabel and skos:hiddenLabel is the
class of RDF plain literals. |
| S13 | skos:prefLabel , skos:altLabel and
skos:hiddenLabel are pairwise disjoint properties. |
| S14 | A resource has no more than one value of
skos:prefLabel per language tag. |
The following graph is consistent, and illustrates the provision of lexical labels in two different languages (French and English).
| Example 10 (consistent) |
|---|
|
<MyResource>
skos:prefLabel "animals"@en ; skos:altLabel "fauna"@en ; skos:hiddenLabel "aminals"@en ; skos:prefLabel "animaux"@fr ; skos:altLabel "faune"@fr . |
The following graph is consistent and illustrates the provision of lexical labels in four different variations (Japanese written with kanji, the hiragana script, the katakana script or with latin characters (rōmaji)).
| Example 11 (consistent) |
|---|
|
<AnotherResource>
skos:prefLabel "東"@ja-Hani ; skos:prefLabel "ひがし"@ja-Hira ; skos:altLabel "あずま"@ja-Hira ; skos:prefLabel "ヒガシ"@ja-Kana ; skos:altLabel "アズマ"@ja-Kana ; skos:prefLabel "higashi"@ja-Latn ; skos:altLabel "azuma"@ja-Latn . |
The following graph is not consistent with the SKOS data model, because two different preferred lexical labels have been given with the same language tag.
| Example 12 (not consistent) |
|---|
|
<Love> skos:prefLabel "love"@en ; skos:prefLabel "adoration"@en
.
|
The following graph is not consistent with the SKOS data model, because there is a clash between the preferred and alternative lexical labels.
| Example 13 (not consistent) |
|---|
|
<Love> skos:prefLabel "love"@en ; skos:altLabel "love"@en
.
|
The following graph is not consistent with the SKOS data model, because there is a clash between alternative and hidden lexical labels.
| Example 14 (not consistent) |
|---|
|
<Love> skos:altLabel "love"@en ; skos:hiddenLabel "love"@en
.
|
The following graph is not consistent with the SKOS data model, because there is a clash between preferred and hidden lexical labels.
| Example 15 (not consistent) |
|---|
|
<Love> skos:prefLabel "love"@en ; skos:hiddenLabel "love"@en
.
|
Note that no domain is stated for
skos:prefLabel
, skos:altLabel
and
skos:hiddenLabel
. Thus, the effective domain of these properties
is the class of all resources (rdfs:Resource
).
Therefore, using the properties skos:prefLabel
,
skos:altLabel
and skos:hiddenLabel
to label
any type of resource is consistent with the SKOS
data model.
In the example graph below, skos:prefLabel
,
skos:altLabel
and skos:hiddenLabel
have been used
to label a resource of type owl:Class
— this is
consistent with the SKOS data model.
| Example 16 (consistent) |
|---|
|
<MyClass> rdf:type owl:Class ;
skos:prefLabel "animals"@en ; skos:altLabel "fauna"@en ; skos:hiddenLabel "aminals"@en ; skos:prefLabel "animaux"@fr ; skos:altLabel "faune"@fr . |
Note that the range of skos:prefLabel
,
skos:altLabel
and skos:hiddenLabel
is the class of
RDF plain literals [RDF-CONCEPTS].
By convention, RDF plain literals are always used in the object position
of a triple, where the predicate is one of skos:prefLabel
,
skos:altLabel
or skos:hiddenLabel
. If a
graph does not follow this usage convention an
application may reject such data but is not required to. See also the
note below.
Some applications require additional functionality relating to labels, for example allowing the description of those labels or the definition of additional relations between the labels (such as acronyms). This can be achieved through the identification of labels using URIs. The SKOS eXtension for Labels defined in Appendix A provides support for this.
In the graph below, a resource has two alternative lexical labels, but no preferred lexical label. This is consistent with the SKOS data model, and there are no additional entailments which follow from the data model. However, note that many applications will require a preferred lexical label in order to generate an optimum human-readable display.
| Example 17 (consistent) |
|---|
|
<Love> skos:altLabel "adoration"@en , "desire"@en .
|
[BCP47] defines tags for identifying languages. Note that "en", "en-GB", "en-US" are three different language tags, used with English, British English and US English respectively. Similarly "ja", "ja-Hani", "ja-Hira", "ja-Kana" and "ja-Latn" are five different language tags used with Japanese, Japanese written with kanji, the hiragana script, the katakana script or with latin characters (rōmaji) respectively.
The graph below is consistent with the SKOS data model, because "en", "en-US" and "en-GB" are different language tags.
| Example 18 (consistent) |
|---|
|
<Colour> skos:prefLabel "color"@en , "color"@en-US ,
"colour"@en-GB .
|
In the graph below, there is no clash between the lexical labeling properties, again because "en" and "en-GB" are different language tags, and therefore the graph is consistent with the SKOS data model.
| Example 19 (consistent) |
|---|
|
<Love> skos:prefLabel "love"@en ; skos:altLabel "love"@en-GB
.
|
Note however that, as stated above in section 5.1, these examples serve only to illustrate general features of the SKOS data model, and do not necessarily indicate best practice for the provision of labels with different language tags. Application- and language-specific usage conventions with respect to labels and language tags are out of scope for the SKOS Reference.
It is suggested that applications match requests for labels in a given language to labels with related language tags that are provided by a SKOS concept scheme, e.g., by implementing the "lookup" algorithm defined by [BCP 47]. Applications that perform matching in this way do not require labels to be provided in all possible language variations (of which there could be many), and are compatible with SKOS concept schemes that provide only those labels whose lexical forms are distinct for a given language or collection of languages.
A notation is a string of characters such as "T58.5" or "303.4833" used to uniquely identify a concept within the scope of a given concept scheme.
A notation is different from a lexical label in that a notation is not normally recognizable as a word or sequence of words in any natural language.
This section defines the skos:notation
property. This
property is used to assign a notation as a typed literal
[RDF-CONCEPTS].
skos:notation |
| S15 |
skos:notation is an instance of
owl:DatatypeProperty . |
The example below illustrates a resource
<http://example.com/ns/MyConcept>
with a notation whose
lexical form is the UNICODE string "303.4833" and whose datatype is denoted
by the URI <http://example.com/ns/MyNotationDatatype>
.
| Example 20 (consistent) |
|---|
|
<MyConcept> skos:notation
"303.4833"^^<MyNotationDatatype> .
|
A typed literal is a UNICODE string combined with a datatype URI [RDF-CONCEPTS].
Typed literals are commonly used to denote values such as integers,
floating point numbers and dates, and there are a number of datatypes
pre-defined by the XML Schema specification [XML-SCHEMA] such as
xs:integer
, xs:float
and xs:date
.
For other situations, new datatypes can be defined, and these are commonly called "user-defined datatypes" [SWBP-DATATYPES].
By convention, the property skos:notation
is only used with a
typed literal in the object position of the triple, where the datatype URI
denotes a user-defined datatype corresponding to a particular system of
notations or classification codes.
For many situations it may be sufficient to simply coin a datatype URI for a particular notation system, and define the datatype informally via a document that describes how the notations are constructed and/or which lexical forms are allowed. Note, however, that it is also possible to define at least the lexical space of a datatype more formally via the XML Schema language, see [SWBP-DATATYPES] section 2. Users should be aware that tools may vary in their support of datatypes. However, as discussed in [OWL-REFERENCE] section 6.3, tools should at least treat lexically identical literals as equal.
There are no constraints on the cardinality of the
skos:notation
property. A concept may have zero, 1 or more
notations.
Where a concept has more than 1 notation, these may be from the same or different notation systems. In the case where notations are from different systems, different datatypes may be used to indicate this. It is not common practice to assign more than one notation from the same notation system (i.e., with the same datatype URI).
By convention, no two concepts in the same concept scheme are given the same notation. If they were, it would not be possible to use the notation to uniquely refer to a concept (i.e., the notation would become ambiguous).
There are no constraints on the combined use of skos:notation
and skos:prefLabel
. In the example below, the same string is
given both as the lexical form of a notation and as a the lexical form of a
preferred label.
| Example 21 (consistent) |
|---|
|
<Potassium>
skos:prefLabel "K"@en ; skos:notation "K"^^<ChemicalSymbolNotation> . |
Typed literals consist of a string of characters and a datatype
URI. By convention, skos:notation
is only used
with typed literals in the object position of the
triple.
Plain literals consist of a string of characters and a language tag. By
convention, skos:prefLabel
(and skos:altLabel
and
skos:hiddenLabel
) are only used with plain
literals in the object position of the triple.
There is no such thing as an RDF literal with both a language tag and a datatype URI, i.e., a typed literal does not have a language tag, and a plain literal does not have a datatype URI.
Note that no domain is stated for skos:notation
. Thus, the
effective domain is the class of all resources (rdfs:Resource
).
Therefore, using skos:notation
with any type of resource is
consistent with the SKOS data model.
Notes are used to provide information relating to SKOS concepts. There is no restriction on the nature of this information, e.g., it could be plain text, hypertext, or an image; it could be a definition, information about the scope of a concept, editorial information, or any other type of information.
There are seven properties in SKOS for associating notes with concepts, defined formally in this section. For more information on the recommended usage of each of the SKOS documentation properties, see the [SKOS-PRIMER].
These seven properties are not intended to cover every situation, but rather to be useful in some of the most common situations, and to provide a set of extension points for defining more specific types of note. For more information on recommended best practice for extending SKOS, see the [SKOS-PRIMER].
Three different usage patterns are recommended in the [SKOS-PRIMER] for the SKOS documentation properties — "documentation as an RDF literal", "documentation as a related resource description" and "documentation as a document reference". The data model defined in this section is intended to accommodate all three design patterns.
skos:note |
skos:changeNote |
skos:definition |
skos:editorialNote |
skos:example |
skos:historyNote |
skos:scopeNote |
| S16 |
skos:note , skos:changeNote ,
skos:definition , skos:editorialNote ,
skos:example , skos:historyNote and
skos:scopeNote are each instances of
owl:AnnotationProperty . |
| S17 | skos:changeNote , skos:definition ,
skos:editorialNote , skos:example ,
skos:historyNote and skos:scopeNote are
each sub-properties of skos:note . |
The graph below gives an example of the "documentation as an RDF literal" pattern.
| Example 22 (consistent) |
|---|
|
<MyResource> skos:note "this is a note"@en .
|
The graph below gives an example of the "documentation as a document reference" pattern.
| Example 23 (consistent) |
|---|
|
<MyResource> skos:note <MyNote> .
|
Note that no domain is stated for the SKOS documentation
properties. Thus, the effective domain for these properties is the class of
all resources (rdfs:Resource
). Therefore, using the SKOS
documentation properties to provide information on any type of
resource is consistent with the SKOS data model.
In the example graph below, skos:definition
has been
used to provide a plain text definition for a resource of type
owl:Class
— this is consistent with the SKOS data model.
| Example 24 (consistent) |
|---|
|
<Protein> rdf:type owl:Class ;
skos:definition """A physical entity consisting of a sequence of amino-acids; a protein monomer; a single polypeptide chain. An example is the EGFR protein."""@en . |
Note that no range is stated for the SKOS documentation properties,
and thus the range of these properties is effectively the class of all
resources (rdfs:Resource
). Under the RDF and OWL Full semantics,
everything is a resource, including RDF plain literals.
SKOS semantic relations are links between SKOS concepts, where the link is inherent in the meaning of the linked concepts.
The Simple Knowledge Organization System distinguishes between two basic categories of semantic relation: hierarchical and associative. A hierarchical link between two concepts indicates that one is in some way more general ("broader") than the other ("narrower"). An associative link between two concepts indicates that the two are inherently "related", but that one is not in any way more general than the other.
The properties skos:broader
and skos:narrower
are used to assert a direct hierarchical link between two SKOS concepts. A
triple <A> skos:broader <B>
asserts that
<B>
, the object of the triple, is a broader concept than
<A>
, the subject of the triple. Similarly, a triple
<C> skos:narrower <D>
asserts that
<D>
, the object of the triple, is a narrower concept than
<C>
, the subject of the triple.
By convention, skos:broader
and skos:narrower
are only used to assert a direct (i.e.,
immediate) hierarchical link between two SKOS concepts. This provides
applications with a convenient and reliable way to access the direct broader
and narrower links for any given concept. Note that, to support this usage
convention, the properties skos:broader
and
skos:narrower
are not declared as transitive
properties.
Some applications need to make use of both direct and
indirect hierarchical links between concepts, for instance to
improve search recall through query expansion. For this purpose, the
properties skos:broaderTransitive
and
skos:narrowerTransitive
are provided. A triple
<A>
skos:broaderTransitive
<B>
represents a direct or indirect hierarchical link,
where <B>
is a broader "ancestor" of
<A>
. Similarly a triple <C>
skos:narrowerTransitive <D>
represents a direct or indirect
hierarchical link, where <D>
is a narrower "descendant" of
<C>
.
By convention, the properties skos:broaderTransitive
and
skos:narrowerTransitive
are not used to make
assertions. Rather, these properties are used to infer the transitive closure
of the hierarchical links, which can then be used to access direct or
indirect hierarchical links between concepts.
The property skos:related
is used to assert an associative
link between two SKOS concepts.
For more examples of stating hierarchical and associative links, see the [SKOS-PRIMER].
skos:semanticRelation |
skos:broader |
skos:narrower |
skos:related |
skos:broaderTransitive |
skos:narrowerTransitive |
| S18 |
skos:semanticRelation ,
skos:broader , skos:narrower ,
skos:related , skos:broaderTransitive and
skos:narrowerTransitive are each instances of
owl:ObjectProperty . |
| S19 | The rdfs:domain of skos:semanticRelation
is the class skos:Concept . |
| S20 | The rdfs:range of skos:semanticRelation
is the class skos:Concept . |
| S21 | skos:broaderTransitive ,
skos:narrowerTransitive and skos:related
are each sub-properties of skos:semanticRelation . |
| S22 | skos:broader is a sub-property of
skos:broaderTransitive , and skos:narrower
is a sub-property of skos:narrowerTransitive . |
| S23 | skos:related is an instance of
owl:SymmetricProperty . |
| S24 | skos:broaderTransitive and
skos:narrowerTransitive are each instances of
owl:TransitiveProperty . |
| S25 | skos:narrower is owl:inverseOf the
property skos:broader . |
| S26 | skos:narrowerTransitive is
owl:inverseOf the property
skos:broaderTransitive . |
| S27 | skos:related is disjoint with the property
skos:broaderTransitive . |
Note that because skos:related
is a symmetric property, and
skos:broaderTransitive
and skos:narrowerTransitive
are inverses, skos:related
is therefore also disjoint with
skos:narrowerTransitive
.
The graph below asserts a direct hierarchical link between
<A>
and <B>
(where
<B>
is broader than <A>
), and an
associative link between <A>
and <C>
,
and is consistent with the SKOS data model.
| Example 25 (consistent) |
|---|
|
<A> skos:broader <B> ; skos:related <C> .
|
The graph below is not consistent with the SKOS data model, because there is a clash between associative links and hierarchical links.
| Example 26 (not consistent) |
|---|
|
<A> skos:broader <B> ; skos:related <B> .
|
The graph below is not consistent with the SKOS data model, again because there is a clash between associative links and hierarchical links.
| Example 27 (not consistent) |
|---|
|
<A> skos:broader <B> ; skos:related <C> .
<B> skos:broader <C> . |
In the example above, the clash is not immediately obvious. The clash becomes apparent when inferences are drawn, based on the class and property definitions above, giving the following graph.
| Example 28 (not consistent) |
|---|
|
<A> skos:broaderTransitive <C> ; skos:related <C>
.
|
The graph below is not consistent with the SKOS data model, again because there is a clash between associative links and hierarchical links, which can be inferred from the class and property definitions given above.
| Example 29 (not consistent) |
|---|
|
<A> skos:narrower <B> ; skos:related <C> .
<B> skos:narrower <C> . |
The diagram below illustrates informally the sub-property relationships between the SKOS semantic relation properties.
skos:semanticRelation | +— skos:related | +— skos:broaderTransitive | | | +— skos:broader | +— skos:narrowerTransitive | +— skos:narrower
Note that the domain and range of skos:semanticRelation
is
the class skos:Concept
. Because skos:broader
,
skos:narrower
and skos:related
are each
sub-properties of skos:semanticRelation
, the graph in example 26
above entails that <A>
, <B>
and
<C>
are each instances of skos:Concept
.
skos:related
is a symmetric property. The example below
illustrates an entailment which follows from this condition.
| Example 30 (entailment) |
|---|
|
<A> skos:related <B> .
entails
<B> skos:related <A> .
|
Note that, although skos:related
is a symmetric property,
this condition does not place any restrictions on
sub-properties of skos:related
(i.e., sub-properties of
skos:related
could be symmetric, not symmetric or antisymmetric,
and still be consistent with the SKOS data model).
To illustrate this point, in the example below, two new properties which
are not symmetric are declared as sub-properties of
skos:related
. The example, which is consistent
with the SKOS data model, also shows some of the entailments which follow.
| Example 31 (entailment) |
|---|
|
<cause> rdf:type owl:ObjectProperty ;
rdfs:subPropertyOf skos:related . <effect> rdf:type owl:ObjectProperty ; rdfs:subPropertyOf skos:related ; owl:inverseOf <cause> . <A> <cause> <B> . entails
<A> skos:related <B> .
<B> <effect> <A> ; skos:related <A> . |
See also the [SKOS-PRIMER] for best practice recommendations on extending SKOS.
Note that skos:related
is not a transitive
property. Therefore, the SKOS data model does not support an
entailment as illustrated in the example below.
| Example 32 (non-entailment) |
|---|
|
<A> skos:related <B> .
<B> skos:related <C> . does not entail
<A> skos:related <C> .
|
Note that this specification does not state that skos:related
is a reflexive property, neither does it
state that skos:related
is an irreflexive property.
Because skos:related
is not defined as an
irreflexive property, the graph below is consistent with the
SKOS data model.
| Example 33 (consistent) |
|---|
|
<A> skos:related <A> .
|
However, for many applications that use knowledge organization systems,
statements of the form X skos:related
X are a potential problem.
Where this is the case, an application may wish to search for such statements
prior to processing SKOS data, although how an application should handle such
statements is not defined in this specification and may vary between
applications.
Note that skos:broader
is not a transitive
property. Similarly, skos:narrower
is not a
transitive property. Therefore, the SKOS data model does not
support an entailment as illustrated in the example below.
| Example 34 (non-entailment) |
|---|
|
<A> skos:broader <B> .
<B> skos:broader <C> . does not entail
<A> skos:broader <C> .
|
However, skos:broader
is a sub-property of
skos:broaderTransitive
, which is a transitive
property. Similarly, skos:narrower
is a sub-property of
skos:narrowerTransitive
, which is a transitive
property. Therefore the SKOS data model does support the
entailments illustrated below.
| Example 35 (entailment) |
|---|
|
<A> skos:broader <B> .
<B> skos:broader <C> . entails
<A> skos:broaderTransitive <B> .
<B> skos:broaderTransitive <C> . <A> skos:broaderTransitive <C> . |
Note especially that, by convention, skos:broader
and
skos:narrower
are only used to assert immediate
(i.e., direct) hierarchical links between two SKOS concepts. By convention,
skos:broaderTransitive
and skos:narrowerTransitive
are not used to make assertions, but are instead used only
to draw inferences.
This pattern allows the information about direct (i.e., immediate) hierarchical links to be preserved, which is necessary for many tasks (e.g., building various types of visual representation of a knowledge organization system), whilst also providing a mechanism for conveniently querying the transitive closure of those hierarchical links (which will include both direct and indirect links), which is useful in other situations (e.g., query expansion algorithms).
Note also that a sub-property of a transitive property is not necessarily transitive.
See also the note on alternative paths below.
Note that this specification makes no statements regarding the reflexive
characteristics of the skos:broader
relationship. It does not
state that skos:broader
is a reflexive
property, neither does it state that
skos:broader
is an irreflexive property. Thus for any graph and
resource <A>
, the triple:
| Example 36 (consistent) |
|---|
|
<A> skos:broader <A> .
|
may or may not be present. This conservative position allows SKOS to be
used to model both KOS where the interpretation of skos:broader
is reflexive (e.g., a direct translation of an inferred OWL sub-class
hierarchy), or KOS where skos:broader
could be considered
irreflexive (as would be appropriate for most thesauri or classification
schemes).
Similarly, there are no assertions made as to the reflexivity or
irreflexivity of skos:narrower
.
However, for many applications that use knowledge organization systems,
statements of the form X skos:broader
X or Y
skos:narrower
Y represent a potential problem. Where this is the
case, an application may wish to search for such statements prior to
processing SKOS data, although how an application should handle such
statements is not defined in this specification and may vary between
applications.
In the graph below, a cycle has been stated in the hierarchical relation.
Note that this graph is consistent with the SKOS data model,
i.e., there is no condition requiring that
skos:broaderTransitive
be irreflexive.
| Example 37 (consistent) |
|---|
|
<A> skos:broader <B> .
<B> skos:broader <A> . |
However, for many applications where knowledge organization systems are
used, a cycle in the hierarchical relation represents a potential problem.
For these applications, computing the transitive closure of
skos:broaderTransitive
then looking for statements of the form
X
skos:broaderTransitive
X is a
convenient strategy for finding cycles in the hierarchical relation. How an
application should handle such statements is not defined in this
specification and may vary between applications.
In the graph below, there are two alternative paths from A to C in the hierarchical relation.
| Example 38 (consistent) |
|---|
|
<A> skos:broader <B> , <C> .
<B> skos:broader <C> . |
In the graph below, there are two alternative paths from A to D in the hierarchical relation.
| Example 39 (consistent) |
|---|
|
<A> skos:broader <B> , <C> .
<B> skos:broader <D> . <C> skos:broader <D> . |
This is a pattern which arises naturally in poly-hierarchical knowledge organization systems.
Both of these graphs are consistent with the SKOS data model, i.e., there is no condition requiring that there be only one path between any two nodes in the hierarchical relation.
This specification treats the hierarchical and associative relations as fundamentally distinct in nature. Therefore a clash between hierarchical and associative links is not consistent with the SKOS data model. The examples above illustrate some situations in which a clash is seen to arise.
This position follows the usual definitions given to hierarchical and associative relations in thesaurus standards [ISO2788] [BS8723-2], and supports common practice in many existing knowledge organization systems.
Note that this specification takes the stronger position that, not only
are the immediate (i.e., direct) hierarchical and associative links disjoint,
but associative links are also disjoint with indirect hierarchical
links. This is captured formally in the integrity condition asserting that
skos:related
and skos:broaderTransitive
are
disjoint properties.
SKOS concept collections are labeled and/or ordered groups of SKOS concepts.
Collections are useful where a group of concepts shares something in common, and it is convenient to group them under a common label, or where some concepts can be placed in a meaningful order.
skos:Collection |
skos:OrderedCollection |
skos:member |
skos:memberList |
| S28 | skos:Collection and
skos:OrderedCollection are each instances of
owl:Class . |
| S29 | skos:OrderedCollection is a sub-class of
skos:Collection . |
| S30 | skos:member and skos:memberList are each
instances of owl:ObjectProperty . |
| S31 | The rdfs:domain of skos:member is the
class skos:Collection . |
| S32 | The rdfs:range of skos:member is the
union of classes skos:Concept and skos:Collection . |
| S33 | The rdfs:domain of skos:memberList is the
class skos:OrderedCollection . |
| S34 | The rdfs:range of skos:memberList is the
class rdf:List . |
| S35 | skos:memberList is an instance of
owl:FunctionalProperty . |
| S36 | For any resource, every item in the list given as the value of the
skos:memberList property is also a value of the
skos:member property. |
| S37 | skos:Collection is disjoint with each of
skos:Concept and skos:ConceptScheme . |
The graph below illustrates a SKOS collection with 3 members.
| Example 40 (consistent) |
|---|
|
<MyCollection> rdf:type skos:Collection ;
skos:member <X> , <Y> , <Z> . |
The graph below illustrates an ordered SKOS collection with 3 members.
Note the use of the Turtle syntax (...)
, representing an RDF
Collection (list).
| Example 41 (consistent) |
|---|
|
<MyOrderedCollection> rdf:type skos:OrderedCollection ;
skos:memberList ( <X> <Y> <Z> ) . |
Statement S36 states the logical relationship between the
skos:memberList
and skos:member
properties. This
relationship means that a collection can be inferred from an ordered
collection. This is illustrated in the example below.
| Example 42 (entailment) |
|---|
|
<MyOrderedCollection> rdf:type skos:OrderedCollection ;
skos:memberList ( <X> <Y> <Z> ) . entails
<MyOrderedCollection> rdf:type skos:Collection ;
skos:member <X> , <Y> , <Z> . |
Note that SKOS does not provide any way to explicitly state that a collection is not ordered.
Note that skos:memberList
is a functional property, i.e., it
does not have more than one value. This is intended to capture within the
SKOS data model that it doesn't make sense for an ordered collection to have
more than one member list. Unfortunately, there is no way to use this
condition as an integrity condition without explicitly stating that two lists
are different objects. In other words, although the graph below is
consistent with the SKOS data model, it entails nonsense (a
list with two first elements and a forked tail).
| Example 43 (entailment) |
|---|
|
<OrderedCollectionResource>
skos:memberList ( <A> <B> ) , ( <X> <Y> ) . entails
<OrderedCollectionResource>
skos:memberList [ rdf:first <A> , <X> ; rdf:rest [ rdf:first <B> ; rdf:rest rdf:nil ] , [ rdf:first <Y> ; rdf:rest rdf:nil ] ] . |
However, as stated in [RDF-SEMANTICS] section 3.3.3, semantic
extensions to RDF may place extra syntactic well-formedness
restrictions on the use of the RDF collection vocabulary
(rdf:first
, rdf:rest
, rdf:nil
)
in order to rule out such graphs.
In the example below, a collection is nested within another collection.
| Example 44 (consistent) |
|---|
|
<MyCollection> rdf:type skos:Collection ;
skos:member <A> , <B> , <MyNestedCollection> . <MyNestedCollection> rdf:type skos:Collection ; skos:member <X> , <Y> , <Z> . |
This example is consistent with the SKOS data
model, because the range of skos:member
is the
union of skos:Concept
and skos:Collection
.
In the SKOS data model, skos:Concept
and
skos:Collection
are disjoint classes. The domain and range of
the SKOS semantic relation properties is skos:Concept
.
Therefore, if any of the SKOS semantic relation properties (e.g.,
skos:narrower
) are used to link to or from a collection, the
graph will not be consistent with the SKOS data model.
This is illustrated in the example below, which is not consistent with the SKOS data model.
| Example 45 (not consistent) |
|---|
|
<A> skos:narrower <B> .
<B> rdf:type skos:Collection . |
Similarly, the graph below is not consistent with the SKOS data model.
| Example 46 (not consistent) |
|---|
|
<A> skos:broader <B> .
<B> rdf:type skos:Collection . |
Similarly, the graph below is not consistent with the SKOS data model.
| Example 47 (not consistent) |
|---|
|
<A> skos:related <B> .
<B> rdf:type skos:Collection . |
However, the graph below is consistent with the SKOS data model.
| Example 48 (consistent) |
|---|
|
<A> skos:narrower <B> , <C> , <D> .
<ResourceCollection> rdfs:label "Resource Collection"@en ; skos:member <B> , <C> , <D> . |
This means that, for thesauri and other knowledge organization systems where node labels are used within the systematic display for that thesaurus, the appropriate SKOS representation requires careful consideration. Furthermore, where node labels are used in the systematic display, it may not always be possible to fully reconstruct the systematic display from a SKOS representation alone. Fully representing all of the information represented in a systematic display of a thesaurus or other knowledge organization system, including details of layout and presentation, is beyond the scope of SKOS.
The SKOS mapping properties are skos:closeMatch
,
skos:exactMatch
, skos:broadMatch
,
skos:narrowMatch
and skos:relatedMatch
. These
properties are used to state mapping (alignment) links between SKOS concepts
in different concept schemes, where the links are inherent in the meaning of
the linked concepts.
The properties skos:broadMatch
and
skos:narrowMatch
are used to state a hierarchical mapping link
between two concepts.
The property skos:relatedMatch
is used to state an
associative mapping link between two concepts.
The property skos:closeMatch
is used to link two concepts
that are sufficiently similar that they can be used interchangeably in
some information retrieval applications. In order to avoid
the possibility of "compound errors" when combining mappings across more than
two concept schemes, skos:closeMatch
is not
declared to be a transitive property.
The property skos:exactMatch
is used to link two concepts,
indicating a high degree of confidence that the concepts can be used
interchangeably across a wide range of information retrieval applications.
skos:exactMatch
is a transitive property, and is a sub-property
of skos:closeMatch
.
skos:mappingRelation |
skos:closeMatch |
skos:exactMatch |
skos:broadMatch |
skos:narrowMatch |
skos:relatedMatch |
| S38 | skos:mappingRelation , skos:closeMatch ,
skos:exactMatch , skos:broadMatch ,
skos:narrowMatch and skos:relatedMatch are
each instances of owl:ObjectProperty . |
| S39 | skos:mappingRelation is a sub-property of
skos:semanticRelation . |
| S40 | skos:closeMatch , skos:broadMatch ,
skos:narrowMatch and skos:relatedMatch are
each sub-properties of skos:mappingRelation . |
| S41 | skos:broadMatch is a sub-property of
skos:broader , skos:narrowMatch is a
sub-property of skos:narrower , and
skos:relatedMatch is a sub-property of
skos:related . |
| S42 | skos:exactMatch is a sub-property of
skos:closeMatch . |
| S43 | skos:narrowMatch is owl:inverseOf the
property skos:broadMatch . |
| S44 | skos:relatedMatch , skos:closeMatch and
skos:exactMatch are each instances of
owl:SymmetricProperty . |
| S45 | skos:exactMatch is an instance of
owl:TransitiveProperty . |
| S46 | skos:exactMatch is disjoint with each of the
properties skos:broadMatch and
skos:relatedMatch . |
Note that because skos:exactMatch
is a symmetric property, and
skos:broadMatch
and skos:narrowMatch
are inverses,
skos:exactMatch
is therefore also disjoint with
skos:narrowMatch
.
The graph below asserts an exact equivalence mapping link between
<A>
and <B>
.
| Example 49 (consistent) |
|---|
|
<A> skos:exactMatch <B> .
|
The graph below asserts a close equivalence mapping link between
<A>
and <B>
.
| Example 50 (consistent) |
|---|
|
<A> skos:closeMatch <B> .
|
The graph below asserts a hierarchical mapping link between
<A>
and <B>
(where
<B>
is broader than <A>
), and an
associative mapping link between <A>
and
<C>
.
| Example 51 (consistent) |
|---|
|
<A> skos:broadMatch <B> ; skos:relatedMatch <C>
.
|
The graph below is not consistent with the SKOS data model, because there is a clash between exact and hierarchical mapping links.
| Example 52 (not consistent) |
|---|
|
<A> skos:exactMatch <B> ; skos:broadMatch <B> .
|
The graph below is not consistent with the SKOS data model, because there is a clash between exact and associative mapping links.
| Example 53 (not consistent) |
|---|
|
<A> skos:exactMatch <B> ; skos:relatedMatch <B>
.
|
By convention, the SKOS mapping properties are only used to link concepts
in different concept schemes. However, note that using the
SKOS semantic relation properties (skos:broader
,
skos:narrower
, skos:related
) to link concepts in
different concept schemes is also
consistent with the SKOS data model (see Section 8).
The mapping properties skos:broadMatch
,
skos:narrowMatch
and skos:relatedMatch
are provided
as a convenience, for situations where the provenance of data is known, and
it is useful to be able to tell at a glance the difference between internal
links within a concept scheme and mapping links between concept schemes.
However, using the SKOS mapping properties is no substitute for the careful management of RDF graphs or the use of provenance mechanisms.
The rationale behind this design is that it is hard to draw an absolute distinction between internal links within a concept scheme and mapping links between concept schemes. This is especially true in an open environment where different people might re-organize concepts into concept schemes in different ways. What one person views as two concept schemes with mapping links between, another might view as one single concept scheme with internal links only. This specification allows both points of view to co-exist, which (it is hoped) will promote flexibility and innovation in the re-use of SKOS data in the Web.
There is therefore an intimate connection between the SKOS semantic
relation properties and the SKOS mapping properties. The property
skos:broadMatch
is a sub-property of skos:broader
,
skos:narrowMatch
is a sub-property of
skos:narrower
, and skos:relatedMatch
is a
sub-property of skos:related
. The full set of sub-property
relationships is illustrated below.
skos:semanticRelation | +- skos:related | | | +- skos:relatedMatch | +- skos:broaderTransitive | | | +- skos:broader | | | +- skos:broadMatch | +- skos:narrowerTransitive | | | +- skos:narrower | | | +- skos:narrowMatch | +- skos:mappingRelation | +- skos:closeMatch | | | +- skos:exactMatch | +- skos:relatedMatch | +- skos:broadMatch | +- skos:narrowMatch
Examples below illustrate some entailments which follow from this
sub-property tree, and from the domain and range of skos:semanticRelation
.
| Example 54 (entailment) |
|---|
|
<A> skos:broadMatch <B> .
entails
<A> skos:mappingRelation <B> .
<A> skos:broader <B> . <A> skos:broaderTransitive <B> . <A> skos:semanticRelation <B> . <A> rdf:type skos:Concept . <B> rdf:type skos:Concept . |
| Example 55 (entailment) |
|---|
|
<A> skos:narrowMatch <B> .
entails
<A> skos:mappingRelation <B> .
<A> skos:narrower <B> . <A> skos:narrowerTransitive <B> . <A> skos:semanticRelation <B> . <A> rdf:type skos:Concept . <B> rdf:type skos:Concept . |
| Example 56 (entailment) |
|---|
|
<A> skos:relatedMatch <B> .
entails
<A> skos:mappingRelation <B> .
<A> skos:related <B> . <A> skos:semanticRelation <B> . <A> rdf:type skos:Concept . <B> rdf:type skos:Concept . |
| Example 57 (entailment) |
|---|
|
<A> skos:exactMatch <B> .
entails
<A> skos:closeMatch <B> .
<A> skos:mappingRelation <B> . <A> skos:semanticRelation <B> . <A> rdf:type skos:Concept . <B> rdf:type skos:Concept . |
Note also that, because different people might re-organize concepts into concept schemes in different ways, a graph might assert mapping links between concepts in the same concept scheme, and there are no formal integrity conditions in the SKOS data model that would make such a graph inconsistent, e.g., the graph below is consistent with the SKOS data model. However, in practice it is expected that such a graph would only ever arise from the merge of two or more graphs from different sources.
| Example 58 (consistent) |
|---|
|
<A> skos:broadMatch <B> ; skos:relatedMatch <C>
.
<A> skos:inScheme <MyScheme> . <B> skos:inScheme <MyScheme> . <C> skos:inScheme <MyScheme> . |
Examples below illustrate "clashes" between hierarchical and associative mapping links, which are not consistent with the SKOS data model (because of the sub-property relationships illustrated above, and because of the data model for SKOS semantic relation properties defined in Section 8).
| Example 59 (not consistent) |
|---|
|
<A> skos:broadMatch <B> ; skos:relatedMatch <B>
.
|
| Example 60 (not consistent) |
|---|
|
<A> skos:narrowMatch <B> ; skos:relatedMatch <B>
.
|
| Example 61 (not consistent) |
|---|
|
<A> skos:broadMatch <B> .
<B> skos:broadMatch <C> . <A> skos:relatedMatch <C> . |
The only SKOS mapping property which is declared as transitive is
skos:exactMatch
. An example entailment is illustrated below:
| Example 62 (entailment) |
|---|
|
<A> skos:exactMatch <B> .
<B> skos:exactMatch <C> . entails
<A> skos:exactMatch <C> .
|
All other SKOS mapping properties are not transitive. Therefore, entailments as illustrated in examples below are not supported by the SKOS data model.
| Example 63 (non-entailment) |
|---|
|
<A> skos:broadMatch <B> .
<B> skos:broadMatch <C> . does not entail
<A> skos:broadMatch <C> .
|
| Example 64 (non-entailment) |
|---|
|
<A> skos:relatedMatch <B> .
<B> skos:relatedMatch <C> . does not entail
<A> skos:relatedMatch <C> .
|
| Example 65 (non-entailment) |
|---|
|
<A> skos:closeMatch <B> .
<B> skos:closeMatch <C> . does not entail
<A> skos:closeMatch <C> .
|
None of the SKOS mapping properties are reflexive, neither are they irreflexive.
Because skos:exactMatch
, skos:broadMatch
and
skos:relatedMatch
are not
irreflexive, the graph below is consistent
with the SKOS data model.
| Example 66 (consistent) |
|---|
|
<A> skos:exactMatch <A> .
<B> skos:broadMatch <B> . <C> skos:relatedMatch <C> . |
However, see also Section 8 on the reflexivity of SKOS semantic relation properties.
There are no formal integrity conditions preventing either cycles or alternative paths in a graph of hierarchical mapping links.
In the graph below there are two cycles involving
skos:broadMatch
. This graph is consistent with
the SKOS data model.
| Example 67 (consistent) |
|---|
|
<A> skos:broadMatch <B> .
<B> skos:broadMatch <A> . <X> skos:broadMatch <Y> . <Y> skos:broadMatch <Z> . <Z> skos:broadMatch <X> . |
In the graph below there are two alternative paths involving
skos:broadMatch
. This graph is consistent with
the SKOS data model.
| Example 68 (consistent) |
|---|
|
<A> skos:broadMatch <B> .
<B> skos:broadMatch <C> . <A> skos:broadMatch <C> . |
See however Section 8 on cycles and
alternative paths involving skos:broader
.
| Example 69 (entailment) |
|---|
|
<A> skos:exactMatch <B>
entails
<A> skos:exactMatch <A> .
<A> skos:closeMatch <A> . |
Due to the entailment above (which arises through a combination
of S42, S44
and S45), applications must be able to cope with
cycles in skos:exactMatch
and skos:closeMatch
.
There are no sub-property chain axioms in the SKOS data model involving the
skos:exactMatch
or skos:closeMatch
properties.
Hence the entailments illustrated below are not
supported.
| Example 70 (non-entailment) |
|---|
|
<A> skos:exactMatch <B> .
<B> skos:broadMatch <C> . does not entail
<A> skos:broadMatch <C> .
|
| Example 71 (non-entailment) |
|---|
|
<A> skos:exactMatch <B> .
<B> skos:relatedMatch <C> . does not entail
<A> skos:relatedMatch <C> .
|
| Example 72 (non-entailment) |
|---|
|
<A> skos:closeMatch <B> .
<B> skos:broadMatch <C> . does not entail
<A> skos:broadMatch <C> .
|
| Example 73 (non-entailment) |
|---|
|
<A> skos:closeMatch <B> .
<B> skos:relatedMatch <C> . does not entail
<A> skos:relatedMatch <C> .
|
OWL provides three properties which might, at first glance, appear similar
to skos:closeMatch
or skos:exactMatch
.
owl:sameAs
is used to link two individuals in an ontology, and
indicates that they are the same individual (i.e., the same resource).
owl:equivalentClass
is used to link two classes in an ontology,
and indicates that those classes have the same class extension.
owl:equivalentProperty
is used to link two properties in an
ontology and indicates that both properties have the same property
extension.
skos:closeMatch
and skos:exactMatch
are used to
link SKOS concepts in different schemes. A skos:closeMatch
link
indicates that two concepts are sufficiently similar that they can be used
interchangeably in some information retrieval applications.
A skos:exactMatch
link indicates a high degree of confidence
that two concepts can be used interchangeably across a wide range of
information retrieval applications.
owl:sameAs
, owl:equivalentClass
or
owl:equivalentProperty
would typically be inappropriate for
linking SKOS concepts in different concept schemes, because the formal
consequences that follow could be undesirable.
The example below illustrates some undesirable entailments that would
follow from using owl:sameAs
in this way.
| Example 74 (entailment) |
|---|
|
<A> rdf:type skos:Concept ;
skos:prefLabel "love"@en ; skos:inScheme <MyScheme> . <B> rdf:type skos:Concept ; skos:prefLabel "adoration"@en ; skos:inScheme <AnotherScheme> . <A> owl:sameAs <B> . entails
<A>
skos:prefLabel "love"@en ; skos:prefLabel "adoration"@en ; skos:inScheme <MyScheme> ; skos:inScheme <AnotherScheme> . <B> skos:prefLabel "love"@en ; skos:prefLabel "adoration"@en ; skos:inScheme <MyScheme> ; skos:inScheme <AnotherScheme> . |
In this example, using owl:sameAs
to link two SKOS concepts
in different concept schemes does actually lead to an inconsistency with the
SKOS data model, because both <A>
and
<B>
now have two preferred lexical labels in the same
language. This will not always be the case, however.
This document is the result of extensive discussions within the W3C Semantic Web Deployment Working Group. The document drew on the experiences of earlier groups and projects, including SWAD-Europe and the W3C Semantic Web Best Practices and Deployment Working Group. Members of the W3C's public-esw-thes mailing list also made valuable contributions.
| skos:Collection | |
|---|---|
| URI: | http://www.w3.org/2004/02/skos/core#Collection |
| Definition: | Section 9. Concept Collections |
| Label: | Collection |
| Disjoint classes: | skos:Concept skos:ConceptScheme |
| skos:Concept | |
| URI: | http://www.w3.org/2004/02/skos/core#Concept |
| Definition: | Section 3. The skos:Concept Class |
| Label: | Concept |
| Disjoint classes: | skos:Collection skos:ConceptScheme |
| skos:ConceptScheme | |
| URI: | http://www.w3.org/2004/02/skos/core#ConceptScheme |
| Definition: | Section 4. Concept Schemes |
| Label: | Concept Scheme |
| Disjoint classes: | skos:Collection skos:Concept |
| skos:OrderedCollection | |
| URI: | http://www.w3.org/2004/02/skos/core#OrderedCollection |
| Definition: | Section 9. Concept Collections |
| Label: | Ordered Collection |
| Super-classes: | skos:Collection |
| skos:altLabel | |
|---|---|
| URI: | http://www.w3.org/2004/02/skos/core#altLabel |
| Definition: | Section 5. Lexical Labels |
| Label: | alternative label |
| Super-properties: | http://www.w3.org/2000/01/rdf-schema#label |
| skos:broadMatch | |
| URI: | http://www.w3.org/2004/02/skos/core#broadMatch |
| Definition: | Section 10. Mapping Properties |
| Label: | has broader match |
| Super-properties: | skos:broader skos:mappingRelation |
| Inverse of: | skos:narrowMatch |
| skos:broader | |
| URI: | http://www.w3.org/2004/02/skos/core#broader |
| Definition: | Section 8. Semantic Relations |
| Label: | has broader |
| Super-properties: | skos:broaderTransitive |
| Inverse of: | skos:narrower |
| skos:broaderTransitive | |
| URI: | http://www.w3.org/2004/02/skos/core#broaderTransitive |
| Definition: | Section 8. Semantic Relations |
| Label: | has broader transitive |
| Super-properties: | skos:semanticRelation |
| Inverse of: | skos:narrowerTransitive |
| Other characteristics: | Transitive |
| skos:changeNote | |
| URI: | http://www.w3.org/2004/02/skos/core#changeNote |
| Definition: | Section 7. Documentation Properties |
| Label: | change note |
| Super-properties: | skos:note |
| skos:closeMatch | |
| URI: | http://www.w3.org/2004/02/skos/core#closeMatch |
| Definition: | Section 10. Mapping Properties |
| Label: | has close match |
| Super-properties: | skos:mappingRelation |
| Other characteristics: | Symmetric |
| skos:definition | |
| URI: | http://www.w3.org/2004/02/skos/core#definition |
| Definition: | Section 7. Documentation Properties |
| Label: | definition |
| Super-properties: | skos:note |
| skos:editorialNote | |
| URI: | http://www.w3.org/2004/02/skos/core#editorialNote |
| Definition: | Section 7. Documentation Properties |
| Label: | editorial note |
| Super-properties: | skos:note |
| skos:exactMatch | |
| URI: | http://www.w3.org/2004/02/skos/core#exactMatch |
| Definition: | Section 10. Mapping Properties |
| Label: | has exact match |
| Super-properties: | skos:closeMatch |
| Other characteristics: | Transitive Symmetric |
| skos:example | |
| URI: | http://www.w3.org/2004/02/skos/core#example |
| Definition: | Section 7. Documentation Properties |
| Label: | example |
| Super-properties: | skos:note |
| skos:hasTopConcept | |
| URI: | http://www.w3.org/2004/02/skos/core#hasTopConcept |
| Definition: | Section 4. Concept Schemes |
| Label: | label |
| Domain: | skos:ConceptScheme |
| Range: | skos:Concept |
| Inverse of: | skos:topConceptOf |
| URI: | http://www.w3.org/2004/02/skos/core#hiddenLabel |
| Definition: | Section 5. Lexical Labels |
| Label: | hidden label |
| Super-properties: | http://www.w3.org/2000/01/rdf-schema#label |
| skos:historyNote | |
| URI: | http://www.w3.org/2004/02/skos/core#historyNote |
| Definition: | Section 7. Documentation Properties |
| Label: | history note |
| Super-properties: | skos:note |
| skos:inScheme | |
| URI: | http://www.w3.org/2004/02/skos/core#inScheme |
| Definition: | Section 4. Concept Schemes |
| Label: | is in scheme |
| Range: | skos:ConceptScheme |
| skos:mappingRelation | |
| URI: | http://www.w3.org/2004/02/skos/core#mappingRelation |
| Definition: | Section 10. Mapping Properties |
| Label: | is in mapping relation with |
| Super-properties: | skos:semanticRelation |
| skos:member | |
| URI: | http://www.w3.org/2004/02/skos/core#member |
| Definition: | Section 9. Concept Collections |
| Label: | has member |
| Domain: | skos:Collection |
| Range: | union of skos:Concept and skos:Collection |
| skos:memberList | |
| URI: | http://www.w3.org/2004/02/skos/core#memberList |
| Definition: | Section 9. Concept Collections |
| Label: | has member list |
| Domain: | skos:OrderedCollection |
| Range: | http://www.w3.org/1999/02/22-rdf-syntax-ns#List |
| Other characteristics: | Functional |
| skos:narrowMatch | |
| URI: | http://www.w3.org/2004/02/skos/core#narrowMatch |
| Definition: | Section 10. Mapping Properties |
| Label: | has narrower match |
| Super-properties: | skos:mappingRelation skos:narrower |
| Inverse of: | skos:broadMatch |
| skos:narrower | |
| URI: | http://www.w3.org/2004/02/skos/core#narrower |
| Definition: | Section 8. Semantic Relations |
| Label: | has narrower |
| Super-properties: | skos:narrowerTransitive |
| Inverse of: | skos:broader |
| skos:narrowerTransitive | |
| URI: | http://www.w3.org/2004/02/skos/core#narrowerTransitive |
| Definition: | Section 8. Semantic Relations |
| Label: | has narrower transitive |
| Super-properties: | skos:semanticRelation |
| Inverse of: | skos:broaderTransitive |
| Other characteristics: | Transitive |
| skos:notation | |
| URI: | http://www.w3.org/2004/02/skos/core#notation |
| Definition: | Section 6. Notations |
| Label: | notation |
| skos:note | |
| URI: | http://www.w3.org/2004/02/skos/core#note |
| Definition: | Section 7. Documentation Properties |
| Label: | note |
| skos:prefLabel | |
| URI: | http://www.w3.org/2004/02/skos/core#prefLabel |
| Definition: | Section 5. Lexical Labels |
| Label: | preferred label |
| Super-properties: | http://www.w3.org/2000/01/rdf-schema#label |
| skos:related | |
| URI: | http://www.w3.org/2004/02/skos/core#related |
| Definition: | Section 8. Semantic Relations |
| Label: | has related |
| Super-properties: | skos:semanticRelation |
| Other characteristics: | Symmetric |
| skos:relatedMatch | |
| URI: | http://www.w3.org/2004/02/skos/core#relatedMatch |
| Definition: | Section 10. Mapping Properties |
| Label: | has related match |
| Super-properties: | skos:mappingRelation skos:related |
| Other characteristics: | Symmetric |
| skos:scopeNote | |
| URI: | http://www.w3.org/2004/02/skos/core#scopeNote |
| Definition: | Section 7. Documentation Properties |
| Label: | scope note |
| Super-properties: | skos:note |
| skos:semanticRelation | |
| URI: | http://www.w3.org/2004/02/skos/core#semanticRelation |
| Definition: | Section 8. Semantic Relations |
| Label: | is in semantic relation with |
| Domain: | skos:Concept |
| Range: | skos:Concept |
| skos:topConceptOf | |
| URI: | http://www.w3.org/2004/02/skos/core#topConceptOf |
| Definition: | Section 4. Concept Schemes |
| Label: | is top concept in scheme |
| Super-properties: | skos:inScheme |
| Inverse of: | skos:hasTopConcept |
This appendix defines an optional extension to the Simple Knowledge Organization System, called the SKOS eXtension for Labels (SKOS-XL). This extension provides additional support for identifying, describing and linking lexical entities.
A special class of lexical entities, called skosxl:Label
, is
defined. Each instance of this class has a single RDF plain literal form, but
two instances of this class are not necessarily the same individual if they
share the same literal form.
Three labeling properties, skosxl:prefLabel
,
skosxl:altLabel
and skosxl:hiddenLabel
, are
defined. These properties are used to label SKOS concepts with instances of
skosxl:Label
, and are otherwise analogous to the properties of
the same local name defined in SKOS (skos:prefLabel
,
skos:altLabel
and skos:hiddenLabel
respectively).
The SKOS data model also defines the property
skosxl:labelRelation
. This property can be used to assert a
direct (binary) link between instances of skosxl:Label
. It is
primarily intended as an extension point, to be refined for more specific
types of link. No built-in refinements of skosxl:labelRelation
are provided, although some examples of how this could be done are given.
The SKOS-XL namespace URI is:
Here the prefix skosxl:
is used as an abbreviation for the SKOS-XL
namespace URI.
The SKOS-XL vocabulary is the set of URIs given in the left-hand column of the table below.
| URI | Defined by (section of this appendix) |
|---|---|
skosxl:Label |
The skosxl:Label Class |
skosxl:literalForm |
The skosxl:Label Class |
skosxl:prefLabel |
Preferred, Alternate and Hidden skosxl:Labels |
skosxl:altLabel |
Preferred, Alternate and Hidden skosxl:Labels |
skosxl:hiddenLabel |
Preferred, Alternate and Hidden skosxl:Labels |
skosxl:labelRelation |
Links Between skosxl:Labels |
Here "the SKOS-XL vocabulary" refers to the union of the SKOS vocabulary and the SKOS-XL vocabulary.
Here "the XL data model" refers to the class and property definitions stated in this appendix only. "The SKOS+XL data model" refers to the union of the data model defined in sections 3-10 above and the XL data model.
The class skosxl:Label
is a special class of lexical
entities.
An instance of the class skosxl:Label
is a resource and may
be named with a URI.
An instance of the class skosxl:Label
has a single literal
form. This literal form is an RDF plain literal (which is a string of UNICODE
characters and an optional language tag [RDF-CONCEPTS]). The property
skosxl:literalForm
is used to give the literal form of an
skosxl:Label
.
If two instances of the class skosxl:Label
have the same
literal form, they are not necessarily the same resource.
| S47 | skosxl:Label is an instance of
owl:Class . |
| S48 | skosxl:Label is disjoint with each of
skos:Concept , skos:ConceptScheme and
skos:Collection . |
| S49 | skosxl:literalForm is an instance of
owl:DatatypeProperty . |
| S50 | The rdfs:domain of skosxl:literalForm is
the class skosxl:Label . |
| S51 | The rdfs:range of skosxl:literalForm is
the class of RDF plain literals. |
| S52 | skosxl:Label is a sub-class of a restriction on
skosxl:literalForm cardinality exactly 1. |
The example below describes a skosxl:Label
named with the
URI <http://example.com/ns/A>
, with the literal form
"love" in English.
| Example 75 (consistent) |
|---|
|
<A> rdf:type skosxl:Label ; skosxl:literalForm "love"@en .
|
The four examples below are each not consistent with the
XL data model, because an skosxl:Label
is described with two
different literal forms.
| Example 76 (not consistent) |
|---|
|
<B> rdf:type skosxl:Label ; skosxl:literalForm "love" ;
skosxl:literalForm "adoration" .
|
| Example 77 (not consistent) |
|---|
|
<B> rdf:type skosxl:Label ; skosxl:literalForm "love"@en ;
skosxl:literalForm "love"@fr .
|
| Example 78 (not consistent) |
|---|
|
<B> rdf:type skosxl:Label ; skosxl:literalForm "love"@en-GB ;
skosxl:literalForm "love"@en-US .
|
| Example 79 (not consistent) |
|---|
|
<B> rdf:type skosxl:Label ; skosxl:literalForm "東"@ja-Hani ;
skosxl:literalForm "ひがし"@ja-Hira .
|
As stated above, each instance of the class skosxl:Label
has
one and only one literal form. In other words, there is a
function mapping the class extension of skosxl:Label
to the set
of RDF plain literals. This function is defined by the property extension of
skosxl:literalForm
. Note especially two facts about this
function.
First, the function is not injective. In other words,
there is not a one-to-one mapping from instances of
skosxl:Label
to the set of RDF plain literals (in fact it is
many-to-one). This means that two instances of skosxl:Label
which have the same literal form are not necessarily the
same individual. So, for example, the entailment illustrated below is
not supported by the XL data model.
| Example 80 (non-entailment) |
|---|
|
<A> skosxl:literalForm "love"@en .
<B> skosxl:literalForm "love"@en . does not entail
<A> owl:sameAs <B> .
|
Second, the function is not surjective. In other words,
for a given plain literal l
, there might not be any instances of
skosxl:Label
with literal form l
.
The membership of an instance of skosxl:Label
within a SKOS
concept scheme can be asserted using the skos:inScheme
property.
| Example 81 (consistent) |
|---|
|
<A> rdf:type skosxl:Label ; skosxl:literalForm "love"@en ;
skos:inScheme <MyScheme> .
|
The three properties skosxl:prefLabel
,
skosxl:altLabel
and skosxl:hiddenLabel
are used to
give the preferred, alternative and hidden labels of a resource respectively,
where those labels are instances of the class skosxl:Label
.
These properties are analogous to the properties of the same local name
defined in the SKOS vocabulary, and there are logical dependencies between
these two sets of properties defined below.
| S53 | skosxl:prefLabel , skosxl:altLabel and
skosxl:hiddenLabel are each instances of
owl:ObjectProperty . |
| S54 | The rdfs:range of each of
skosxl:prefLabel , skosxl:altLabel and
skosxl:hiddenLabel is the class
skosxl:Label . |
| S55 | The property chain (skosxl:prefLabel ,
skosxl:literalForm ) is a sub-property of
skos:prefLabel . |
| S56 | The property chain (skosxl:altLabel ,
skosxl:literalForm ) is a sub-property of
skos:altLabel . |
| S57 | The property chain (skosxl:hiddenLabel ,
skosxl:literalForm ) is a sub-property of
skos:hiddenLabel . |
| S58 | skosxl:prefLabel , skosxl:altLabel and skosxl:hiddenLabel are pairwise disjoint properties. |
The example below illustrates the use of all three XL labeling properties, and is consistent with the SKOS+XL data model.
| Example 82 (consistent) |
|---|
|
<Love>
skosxl:prefLabel <A> ; skosxl:altLabel <B> ; skosxl:hiddenLabel <C> . <A> rdf:type skosxl:Label ; skosxl:literalForm "love"@en . <B> rdf:type skosxl:Label ; skosxl:literalForm "adoration"@en . <C> rdf:type skosxl:Label ; skosxl:literalForm "luv"@en . |
The sub-property chain axioms S55, S56 and S57 support the dumbing-down of XL labels to vanilla SKOS lexical labels via inference. This is illustrated in the example below.
| Example 83 (entailment) |
|---|
|
<Love>
skosxl:prefLabel <A> ; skosxl:altLabel <B> ; skosxl:hiddenLabel <C> . <A> rdf:type skosxl:Label ; skosxl:literalForm "love"@en . <B> rdf:type skosxl:Label ; skosxl:literalForm "adoration"@en . <C> rdf:type skosxl:Label ; skosxl:literalForm "luv"@en . entails
<Love>
skos:prefLabel "love"@en ; skos:altLabel "adoration"@en ; skos:hiddenLabel "luv"@en . |
In Section 5, two integrity conditions were defined
on the basic SKOS labeling properties. First, the properties
skos:prefLabel
, skos:altLabel
and
skos:hiddenLabel
are pairwise disjoint. Second, a resource has
no more than one value of skos:prefLabel
per language. Because
of the sub-property chain axioms defined above, the following four examples,
whilst consistent w.r.t. the XL data model alone, are not
consistent with the SKOS+XL data model.
| Example 84 (not consistent) |
|---|
|
# Two different preferred labels in the same language
<Love> skosxl:prefLabel <A> ; skosxl:prefLabel <B> . <A> skosxl:literalForm "love"@en . <B> skosxl:literalForm "adoration"@en . |
| Example 85 (not consistent) |
|---|
|
# Clash between preferred and alternative labels
<Love> skosxl:prefLabel <A> ; skosxl:altLabel <B> . <A> skosxl:literalForm "love"@en . <B> skosxl:literalForm "love"@en . |
| Example 86 (not consistent) |
|---|
|
# Clash between alternative and hidden labels
<Love> skosxl:altLabel <A> ; skosxl:hiddenLabel <B> . <A> skosxl:literalForm "love"@en . <B> skosxl:literalForm "love"@en . |
| Example 87 (not consistent) |
|---|
|
# Clash between preferred and hidden labels
<Love> skosxl:prefLabel <A> ; skosxl:hiddenLabel <B> . <A> skosxl:literalForm "love"@en . <B> skosxl:literalForm "love"@en . |
This section defines a pattern for representing binary links between
instances of the class skosxl:Label
.
Note that the vocabulary defined in this section is not intended to be used directly, but rather as an extension point which can be refined for more specific labeling scenarios.
| S59 | skosxl:labelRelation is an instance of
owl:ObjectProperty . |
| S60 | The rdfs:domain of skosxl:labelRelation
is the class skosxl:Label . |
| S61 | The rdfs:range of skosxl:labelRelation is
the class skosxl:Label . |
| S62 | skosxl:labelRelation is an instance of
owl:SymmetricProperty . |
The example below illustrates a link between two instances of the class
skosxl:Label
.
| Example 88 (consistent) |
|---|
|
<A> rdf:type skosxl:Label ; skosxl:literalForm "love" .
<B> rdf:type skosxl:Label ; skosxl:literalForm "adoration" . <A> skosxl:labelRelation <B> . |
As mentioned above, the skosxl:labelRelation
property serves
as an extension point, which can be refined for more specific labeling
scenarios.
In the example below, a third party has refined the property
skos:labelRelation
to express acronym relationships, and used it
to express the fact that "FAO" is an acronym for "Food and Agriculture
Organization".
| Example 89 (consistent) |
|---|
|
# First define an extension to skosxl:labelRelation
ex:acronym rdfs:subPropertyOf skosxl:labelRelation . # Now use it <A> rdf:type skosxl:Label ; skosxl:literalForm "FAO"@en . <B> rdf:type skosxl:Label ; skosxl:literalForm "Food and Agriculture Organization"@en . <B> ex:acronym <A> . |
Note that a sub-property of a symmetric property is not necessarily symmetric.
The following table gives an overview of the SKOS-XL vocabulary.
| skosxl:Label | |
|---|---|
| URI: | http://www.w3.org/2008/05/skos-xl#Label |
| Definition: | Section B.2. The skosxl:Label Class |
| Label: | Label |
| Super-classes: | restriction on skosxl:literalForm cardinality exactly 1 |
| Disjoint classes: | skos:Collection skos:Concept skos:ConceptScheme |
| skosxl:altLabel | |
|---|---|
| URI: | http://www.w3.org/2008/05/skos-xl#altLabel |
| Definition: | Section B.3. Preferred, Alternate and Hidden skosxl:Labels |
| Label: | alternative label |
| Range: | skosxl:Label |
| URI: | http://www.w3.org/2008/05/skos-xl#hiddenLabel |
| Definition: | Section B.3. Preferred, Alternate and Hidden skosxl:Labels |
| Label: | hidden label |
| Range: | skosxl:Label |
| skosxl:labelRelation | |
| URI: | http://www.w3.org/2008/05/skos-xl#labelRelation |
| Definition: | Section B.4. Links Between skosxl:Labels |
| Label: | label relation |
| Domain: | skosxl:Label |
| Range: | skosxl:Label |
| Other characteristics: | Symmetric |
| skosxl:literalForm | |
| URI: | http://www.w3.org/2008/05/skos-xl#literalForm |
| Definition: | Section B.2. The skosxl:Label Class |
| Label: | literal form |
| Domain: | skosxl:Label |
| skosxl:prefLabel | |
| URI: | http://www.w3.org/2008/05/skos-xl#prefLabel |
| Definition: | Section B.3. Preferred, Alternate and Hidden skosxl:Labels |
| Label: | preferred label |
| Range: | skosxl:Label |
Following Architecture of the World Wide Web, Volume One [WEBARCH], a "namespace document" is an "information resource that contains useful information, machine-usable and/or human-usable, about terms in the namespace".
The SKOS vocabulary is a conceptual resource identified by the
namespace URI http://www.w3.org/2004/02/skos/core#
. The
normative definition of the SKOS vocabulary is found in SKOS Reference
(this document).
The following namespace documents provide alternative representations of the SKOS vocabulary:
The SKOS vocabulary is summarized in SKOS Namespace Document - HTML
Variant [SKOS-HTML], which is served from the
namespace URI http://www.w3.org/2004/02/skos/core#
via content negotiation using Recipe 3 of "Best Practice Recipes for
Publishing Vocabularies" [RECIPES]. Clients requiring HTML or XHTML
should include an appropriate "Accept" header in the HTTP
request. Alternatively, the SKOS Namespace Document - HTML Variant can
be referenced directly by citing its URI: http://www.w3.org/TR/skos-reference/skos.html
.
The SKOS Namespace Document - HTML Variant replicates Appendix A. SKOS Properties and Classes of this document.
The SKOS Namespace Document - RDF/XML Variant [SKOS-RDF] expresses the SKOS
vocabulary and data model (in so far as possible) as RDF triples. It
is served via content negotiation using Recipe 3 of "Best Practice
Recipes for Publishing Vocabularies" [RECIPES]. Clients requiring
RDF/XML should include an appropriate "Accept" header in the HTTP
request. Alternatively, the RDF schema can be referenced directly
(and downloaded) by citing its URI: http://www.w3.org/TR/skos-reference/skos.rdf
.
It is not possible to express all of the statements of the SKOS data model as RDF triples, so this schema forms a normative subset of SKOS Reference. The RDF schema defines an OWL Full ontology [OWL-SEMANTICS] [OWL-REFERENCE]. SKOS vocabularies can be defined as instances of this ontology.
For the convenience of tools and applications that wish to work within the constraints of OWL DL, the SKOS RDF Schema - OWL 1 DL Sub-set [SKOS-RDF-OWL1-DL] provides a modified, informative, schema which conforms to those constraints. Note that this schema is obtained through the deletion of triples representing axioms that violate OWL DL constraints. Alternative prunings could be performed.
The SKOS OWL 1 DL Sub-set is available
by citing its URI: http://www.w3.org/TR/skos-reference/skos-owl1-dl.rdf
The SKOS-XL vocabulary is summarized in SKOS-XL Namespace Document
- HTML Variant [SKOS-XL-HTML], which is served from the
namespace URI http://www.w3.org/2008/05/skos-xl#
via content negotiation using Recipe 3 of "Best Practice Recipes for
Publishing Vocabularies" [RECIPES]. Clients requiring HTML or XHTML
should include an appropriate "Accept" header in the HTTP
request. Alternatively, the SKOS-XL Namespace Document - HTML Variant
can be referenced directly by citing its URI: http://www.w3.org/TR/skos-reference/skos-xl.html
.
The SKOS-XL HTML Variant Namespace Document replicates Appendix B.5 SKOS-XL Schema Overview of this document.
This RDF schema document expresses the SKOS vocabulary and data
model (in so far as possible) as RDF triples. It is served
from the namespace URI http://www.w3.org/2008/05/skos-xl#
via content
negotiation using Recipe 3 of "Best Practice Recipes for Publishing
Vocabularies" [RECIPES]. Clients requiring RDF/XML should
include an appropriate "Accept" header in the HTTP request.
Alternatively, the RDF schema can be referenced directly (and
downloaded) by citing its URI: http://www.w3.org/TR/skos-reference/skos-xl.rdf
.
The SKOS schema defines vocabulary using the namespace http://www.w3.org/2004/02/skos/core#. This namespace was used to define the original SKOS schema which served as a starting point for this Recommendation. As a result of this, elements present in previous versions of the machine-readable schema have been removed from the current version. In a number of cases, the definition or semantics of elements in the schema has changed.
Retaining the existing SKOS namespace avoids some issues with
existing KOS that are already using the SKOS schema. Users
should, however, be aware of the change in the semantics of
skos:broader
(and skos:narrower
) which may
impact on SKOS applications.
Where elements have been removed, no explicit deprecation axioms have been expressed in the schema. Historical versions of the SKOS schema, are, however, available from the SKOS Web site "version history" page, and those elements which have been removed from the recent version of the vocabulary are listed below.
skos:symbol
skos:prefSymbol
skos:altSymbol
skos:CollectableProperty
skos:subject
skos:isSubjectOf
skos:primarySubject
skos:isPrimarySubjectOf
skos:subjectIndicator
In the case of skos:broader
and
skos:narrower
, the semantics of the vocabulary elements
have been changed — these properties are no longer declared to be
transitive. Thus the follow entailment does not hold.
| Example 90 (non-entailment) |
|---|
|
<A> skos:broader <B> .
<B> skos:broader <C> . does not entail
<A> skos:broader <C> .
|
A transitive super property of skos:broader
—
skos:broaderTransitive
— is provided which allows for
query across the transitive closure of skos:broader
relations. A similar property
— skos:narrowerTransitive
— is provided
for query across the transitive closure of skos:narrower
.