SKOS Core Vocabulary Specification

Abstract

SKOS Core is a model and an RDF vocabulary for expressing the basic structure and content of concept schemes such as thesauri, classification schemes, subject heading lists, taxonomies, 'folksonomies', other types of controlled vocabulary, and also concept schemes embedded in glossaries and terminologies.

The SKOS Core Vocabulary is an application of the Resource Description Framework (RDF), that can be used to express a concept scheme as an RDF graph. Using RDF allows data to be linked to and/or merged with other data, enabling data sources to be distributed across the web, but still be meaningfully composed and integrated.

This document gives a reference-style overview of the SKOS Core Vocabulary as it stands at the time of publication. It also describes the policies for ownership, naming, persistence and change by which the SKOS Core Vocabulary is managed.

This edition of the SKOS Core Vocabulary Specification is a W3C Public Working Draft. It is the authoritative human-readable account of the SKOS Core Vocabulary at the time of publication.

See also the SKOS Core Guide.

Status of this Document

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 Working Draft published by the Semantic Web Best Practices and Deployment Working Group, part of the W3C Semantic Web Activity. The Working Group intends the SKOS Core Vocabulary Specification to become a W3C Working Group Note (see W3C document maturity-levels). However, other outcomes are possible within the framework of the W3C Process and will be considered in response to deployment experience and feedback from the W3C membership. The Working Group has discussed the potential for SKOS Core to evolve into possible future W3C Recommendation Track work items, and would value feedback on the level of formal standardization that is appropriate.

Changes from the previous version are noted in the Release Notes section.

We encourage public comments on this Working Draft. Please send comments to public-esw-thes@w3.org [archive] and start the subject line of the message with "comment:".

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The English language variant of this document is normative, all other language variants are informative.

Contents

• Introduction
• Translations
• Guide to Term Summary Tables
• Policy Statements
  • Ownership
  • Naming
  • Persistence
  • Change
• Release Notes
• Classes
• Properties
• Deprecated
• Acknowledgements

Introduction

SKOS stands for Simple Knowledge Organisation System. The name SKOS was chosen to emphasise the goal of providing a simple yet powerful model for expressing knowledge organisation systems in a machine-understandable way, within the framework of the Semantic Web.

SKOS Core is a model for expressing the basic structure and content of concept schemes such as thesauri, classification schemes, subject heading lists, taxonomies, 'folksonomies', other types of controlled vocabulary, and also concept schemes embedded in glossaries and terminologies.

The SKOS Core Vocabulary is an application of the Resource Description Framework (RDF). RDF provides a simple data formalism for talking about things, their properties, inter-relationships, and categories (classes). Using RDF allows data to be linked to and/or merged with other RDF data by Semantic Web applications. In practice, this means that data sources can be distributed across the web in a decentralised way, but still be meaningfully composed and integrated by applications, often in novel and unanticipated ways. See RDF Concepts for an overview of RDF, RDF Semantics for its formal mathematical basis, and RDF Syntax for details of the RDF/XML document format used to exchange RDF data.

This document gives a reference-style overview of the SKOS Core Vocabulary as it stands at the time of publication. It also describes the policies for ownership, naming, persistence and change by which the SKOS Core Vocabulary is managed.

A formal representation of the SKOS Core Vocabulary is maintained in RDF/OWL [latest: http://www.w3.org/2004/02/skos/core]. Historical snapshots of the RDF/OWL description of the SKOS Core Vocabulary can be obtained from [http://www.w3.org/2004/02/skos/core/history/].

Translations

To obtain the definitive RDF/OWL description of the SKOS Core Vocabulary, serialised as RDF/XML, dereference the URI http://www.w3.org/2004/02/skos/core. This includes labels, comments and definitions in English only, which are normative.

To obtain labels, comments and definitions for classes and properties of the SKOS Core Vocabulary in another language, serialised as RDF/XML, dereference one of the following URIs:

• http://www.w3.org/2004/02/skos/core_de [de] (translated by Thomas Bandholtz)
• http://www.w3.org/2004/02/skos/core_fr [fr] (translated by Bernard Vatant)
• http://www.w3.org/2004/02/skos/core_nl [nl] (translated by Mark van Assem)
• http://www.w3.org/2004/02/skos/core_pt [pt] (translated by Tiago Murakami)

Labels, definitions and comments in any language other than English are informative.

Corresponding language variants of this document are also available at the following links:

• SKOS Core Vocabulary Specification (de)
• SKOS Core Vocabulary Specification (en)
• SKOS Core Vocabulary Specification (fr)
• SKOS Core Vocabulary Specification (nl)
• SKOS Core Vocabulary Specification (pt)

If you would like to submit a translation of labels, definitions and comments, or to make a comment on existing translations, send an email to public-esw-thes@w3.org.

See also the SKOS Core Translations web page.

Guide to Term Summary Tables

Each term (i.e. class or property) of the vocabulary is summarised in this document as a table. Each table may have the following rows:

Class or Property:
URI:	The Universal Resource Identifier.
Label:	A human-readable label.
Definition:	An explanation of the meaning of a class or property.
Comment:	Additional information about meaning and/or proper use.
Example:	An example of the use of a class or property.
Status:	The status (stability level) of the class or property.
Issued:	Date on which the class or property was issued.
Modified:	Date on which the class or property was last modified.
Super-classes:	(Classes only) any declared super-classes.
Super-properties:	(Properties only) any declared super-properties.
Domain:	(Properties only) the declared domain for the property.
Range:	(Properties only) the declared range for the property.
Characteristics:	(Properties only) any declared logical characteristics for the property, e.g. Transitive, Symmetric, Functional, InverseFunctional.
Inverse of:	(Properties only) any declared inverse properties.
Replaces:	Any deprecated terms which the given term has replaced in recommended usage.
Version info:	A note about the modification and/or history of a class or property.
Replaced By:	(Deprecated terms only) the term to use instead of the deprecated term.
Deprecated:	(Deprecated terms only) the date of last modification (i.e. deprecation) of the term.

Policy Statements

N.B. The Working Group is committed to establishing clear expectations around the management of RDF vocabularies, through documentation of process and maintenance policies. This is itself an evolving process. Specifically, this document is itself situated within the W3C Process, and may change and evolve in the light of feedback on SKOS Core and on the SKOS Core policy statements. It should be noted that claims made by the Working Group using the (experimental) persistence and change terminology employed here have as their scope the currently chartered Working Group. They have only draft status within the wider W3C Process. W3C has not delegated to the Working Group any authority to make binding commitments on behalf of W3C beyond those implicit in the formal W3C Process.

Ownership

The W3C gives control over the SKOS Core Vocabulary to working groups within the overall framework of the W3C process. Currently that control resides with the Semantic Web Best Practices and Deployment Working Group, whose chairs have delegated responsibility for maintaining the SKOS Core Vocabulary to the editors of this specification (Alistair Miles and Dan Brickley). When this working group's charter expires, control will revert to W3C as an organization.

The Working Group is committed to a public, consensus-driven design environment for SKOS Core, and to this end conducts SKOS-related discussion in public, in particular drawing on feedback from the Semantic Web Interest Group mailing list public-esw-thes@w3.org.

Naming

The URI for the SKOS Core Vocabulary itself is:

http://www.w3.org/2004/02/skos/core

The URI for a class or property in the SKOS Core Vocabulary is constructed by appending a fragment identifier to the URI for the SKOS Core Vocabulary. E.g.

        
        http://www.w3.org/2004/02/skos/core#Concept
        http://www.w3.org/2004/02/skos/core#prefSymbol
    

A fragment identifier for a class always starts with an uppercase character. Where the fragment identifier is comprised of multiple concatenated words, the leading character of each word will be an uppercase character. E.g.

        Concept
        ConceptScheme
    

The fragment identifier for a property starts with a lowercase character. Where the fragment identifier is comprised of multiple concatenated words, the leading character of the second and each subsequent word will be an uppercase character. E.g.

        subject
        prefLabel
        isPrimarySubjectOf
    

Persistence

All editions of the SKOS Core Vocabulary Specification, all editions of the SKOS Core Guide, and the RDF/OWL description of the SKOS Core Vocabulary [http://www.w3.org/2004/02/skos/core] are all declared to be persistent resources, as defined by the persistence policy at [http://www.w3.org/Consortium/Persistence].

Change

The SKOS Core Vocabulary may change. The process for managing changes to the SKOS Core Vocabulary during the chartered lifetime of the Working Group is described below:

1. The Working Group undertakes to review the SKOS Core Vocabulary Specification and the SKOS Core Guide at intervals of 2 months. Subsequent to each review, new Public Working Draft editions of the SKOS Core Guide and the SKOS Core Vocabulary Specification will be published by the Working Group.
2. In the interim period between publication of Public Working Draft editions of the SKOS Core Guide and the SKOS Core Vocabulary Specification (hereafter 'the interim period'), no changes will be made to the SKOS Core Vocabulary. The formal RDF/OWL description of the SKOS Core Vocabulary [http://www.w3.org/2004/02/skos/core] will therefore also remain unchanged during this period.
3. In the interim period, the editor's will maintain a public list of proposed changes to the SKOS Core Vocabulary to be put forward at the next review.
4. Members of the public may make proposals for change, or comment on current proposals, by sending an email to the publicly archived Semantic Web Interest Group mailing list public-esw-thes@w3.org [archive].
5. Proposed changes to be put forward for review will be published at least 2 weeks before a scheduled review, to allow time for public comment.
6. At each review, the list of proposed changes to the SKOS Core Vocabulary is presented to the reviewers delegated by the Working Group for approval.
7. Those changes approved by the reviewers, or approved in a modified form after negotiation between the editors and the reviewers, will be implemented by the editors. New Public Working Draft editions of the SKOS Core Guide and SKOS Core Vocabulary Specification, reflecting these changes, will then be published by the Working Group. Approved changes will also be implemented in the formal representation of the SKOS Core Vocabulary in RDF/OWL [http://www.w3.org/2004/02/skos/core]. Therefore the formal representation of the SKOS Core Vocabulary in RDF/OWL [http://www.w3.org/2004/02/skos/core] will always be consistent with the latest Public Working Draft editions of the SKOS Core Guide and the SKOS Core Vocabulary Specification.
8. All changes made will also be reported on the SKOS Change Log [http://esw.w3.org/mt/esw/archives/cat_skos_changelog.html].

Furthermore, at any given time only certain types of change are allowed (following the example of the DCMI namespace policy [http://dublincore.org/documents/dcmi-namespace/]). The types of change allowed depend on the 'status' of the class or property to which the change relates. The status of a class or property may take one of three values:

unstable

The term is unstable, and feedback is welcomed on its current form and utility. It may currently be poorly defined. Its meaning and/or form may be expected to change at any time. Do not implement mission critical systems that depend on this term persisting in its current form. (Changes corresponding to DCMI Namespace Policy types A, B or C may occur.)

testing

The term has gone beyond the raw proposal stage, and is undergoing testing. This term may still change in response to feedback from testing, although it may be expected not to undergo any radical change. The cost to early implementors of changing the term will be considered, however the goal of achieving wider interoperability and long-term stability may override those considerations. (Changes corresponding to DCMI Namespace Policy types A or B may occur.)

stable

No substantial (i.e. meaning-changing) alterations will take place. Implementors can expect the term to persist in its current form indefinitely. (Minor editorial changes corresponding to DCMI Namespace Policy type A may occur.)

New classes or properties may be added to the SKOS Core Vocabulary (changes corresponding to DCMI Namespace Policy type D), in accordance with the process described above.

A class or property at any of the above status levels may be marked as deprecated, in which case instructions will be given on what to use in its place. A deprecated class property may be expected to remain unchanged indefinitely.

Historical versions (snapshots) of the SKOS Core Vocabulary are maintained, and can be accessed via the web page at [http://www.w3.org/2004/02/skos/core/history/].

Release Notes

The following changes have been made since the previous (first) W3C Public Working Draft edition of the SKOS Core Vocabulary Specification:

Change 1: Use of skos:subjectIndicator

The property skos:subjectIndicator was previously used internally within the RDF/OWL description of the SKOS Core Vocabulary, to make statements about the other classes and properties of the SKOS Core Vocabulary. This usage, however, has implications which were both inconvenient and contentious, and has been removed. This change should not have any significant impact, and does not alter recommended usage as described in the SKOS Core Guide.

See [subjectIndicatorUse-1] for a description of the original change proposal.

Change 2: Documentation Properties

The properties skos:publicNote and skos:privateNote have been deprecated. They are replaced by the property skos:note. See the Documentation Properties section of the SKOS Core Guide for recommended usage. An example has been added to the Documentation Properties section of the Guide, illustrating the use of the dc:audience property to specify the intended audience of a particular note.

The motivation behind this change is to allow the function of a note to be specified independently from the intended audience, allowing for example different definitions for different audiences.

See [notes-2] for a description of the original change proposal.

Change 3: Range of Symbolic Labelling Properties

The range of the properties skos:prefSymbol and skos:altSymbol has been changed from foaf:Image to dcmitype:Image. This change should not have any significant impact, and has been made purely because of the greater stability of the DCMI type vocabulary at this time.

Also, a new property skos:symbol ('symbolic label') has been added, as the super-property of both skos:prefSymbol ('preferred symbolic label') and skos:altSymbol ('alternative symbolic label'). The skos:symbol property is roughly analagous to rdfs:label, but for labelling a resource with an image resource instead of an RDF literal. The addition of the skos:symbol property does not alter recommended usage of skos:prefSymbol and skos:altSymbol as described in the SKOS Core Guide.

See [symbolicLabelsRange-3] for a description of the original change proposal.

Change 4: Translations

Labels, comments and definitions for the classes and properties of the SKOS Core Vocabulary are now available as RDF in the following languages: French [fr], German [de], Dutch [nl] and Portuguese [pt]. Links have been added from the SKOS Core RDFS/OWL description to these additional sources. Corresponding language variants of this document are also available. This change does not alter recommended usage as described in the SKOS Core Guide.

See [seeAlsoTranslations-4] for a description of the original change proposal.

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Acknowledgements

This work has benefited greatly from contributions made by members of the public-esw-thes@w3.org mailing list. Their feedback, suggestions and encouragement are gratefully acknowledged. SKOS Core draws heavily on conventions and traditions in the thesaurus and library communities, whose practitioners have very generously shared their experiences and insights during the design of SKOS Core.

The following are gratefully acknowledged for their translations: Thomas Bandholtz, Bernard Vatant, Mark van Assem, Tiago Murakami.