# Web Ontology Language

Last updated
Status Published 2004 Mike Dean (BBN Technologies), Guus Schreiber Resource Description Framework, RDFS Semantic Web OWL OWL Reference
Status Published 2009 W3C OWL Working Group Resource Description Framework, RDFS Semantic Web OWL 2 OWL 2 Overview

The Web Ontology Language (OWL) is a family of knowledge representation languages for authoring ontologies. Ontologies are a formal way to describe taxonomies and classification networks, essentially defining the structure of knowledge for various domains: the nouns representing classes of objects and the verbs representing relations between the objects.

## Contents

Ontologies resemble class hierarchies in object-oriented programming but there are several critical differences. Class hierarchies are meant to represent structures used in source code that evolve fairly slowly (perhaps with monthly revisions) whereas ontologies are meant to represent information on the Internet and are expected to be evolving almost constantly. Similarly, ontologies are typically far more flexible as they are meant to represent information on the Internet coming from all sorts of heterogeneous data sources. Class hierarchies on the other hand tend to be fairly static and rely on far less diverse and more structured sources of data such as corporate databases. [1]

The OWL languages are characterized by formal semantics. They are built upon the World Wide Web Consortium's (W3C) XML standard for objects called the Resource Description Framework (RDF). [2] OWL and RDF have attracted significant academic, medical and commercial interest.

In October 2007, [3] a new W3C working group [4] was started to extend OWL with several new features as proposed in the OWL 1.1 member submission. [5] W3C announced the new version of OWL on 27 October 2009. [6] This new version, called OWL 2, soon found its way into semantic editors such as Protégé and semantic reasoners such as Pellet, [7] RacerPro, [8] FaCT++ [9] [10] and HermiT. [11]

The OWL family contains many species, serializations, syntaxes and specifications with similar names. OWL and OWL2 are used to refer to the 2004 and 2009 specifications, respectively. Full species names will be used, including specification version (for example, OWL2 EL). When referring more generally, OWL Family will be used. [12] [13] [14]

## History

### Early ontology languages

There is a long history of ontological development in philosophy and computer science. Since the 1990s, a number of research efforts have explored how the idea of knowledge representation (KR) from artificial intelligence (AI) could be made useful on the World Wide Web. These included languages based on HTML (called SHOE), based on XML (called XOL, later OIL), and various frame-based KR languages and knowledge acquisition approaches.

## Ontology languages for the web

In 2000 in the United States, DARPA started development of DAML led by James Hendler. [15] [ self-published source ] In March 2001, the Joint EU/US Committee on Agent Markup Languages decided that DAML should be merged with OIL. [15] The EU/US ad hoc Joint Working Group on Agent Markup Languages was convened to develop DAML+OIL as a web ontology language. This group was jointly funded by the DARPA (under the DAML program) and the European Union's Information Society Technologies (IST) funding project. DAML+OIL was intended to be a thin layer above RDFS, [15] with formal semantics based on a description logic (DL). [16]

DAML+OIL is a particularly major influence on OWL; OWL's design was specifically based on DAML+OIL. [17]

### Semantic web standards

The Semantic Web provides a common framework that allows data to be shared and reused across application, enterprise, and community boundaries.

World Wide Web Consortium, W3C Semantic Web Activity [18]

#### RDF schema

a declarative representation language influenced by ideas from knowledge representation

World Wide Web Consortium, Metadata Activity [19]

In the late 1990s, the World Wide Web Consortium (W3C) Metadata Activity started work on RDF Schema (RDFS), a language for RDF vocabulary sharing. The RDF became a W3C Recommendation in February 1999, and RDFS a Candidate Recommendation in March 2000. [19] In February 2001, the Semantic Web Activity replaced the Metadata Activity. [19] In 2004 (as part of a wider revision of RDF) RDFS became a W3C Recommendation. [20] Though RDFS provides some support for ontology specification, the need for a more expressive ontology language had become clear. [21] [ self-published source ]

#### Web-Ontology Working Group

As of Monday, the 31st of May, our working group will officially come to an end. We have achieved all that we were chartered to do, and I believe our work is being quite well appreciated.

James Hendler and Guus Schreiber, Web-Ontology Working Group: Conclusions and Future Work [22]

The World Wide Web Consortium (W3C) created the Web-Ontology Working Group as part of their Semantic Web Activity. It began work on November 1, 2001 with co-chairs James Hendler and Guus Schreiber. [22] The first working drafts of the abstract syntax, reference and synopsis were published in July 2002. [22] OWL became a formal W3C recommendation on February 10, 2004 and the working group was disbanded on May 31, 2004. [22]

#### OWL Working Group

In 2005, at the OWL Experiences And Directions Workshop a consensus formed that recent advances in description logic would allow a more expressive revision to satisfy user requirements more comprehensively whilst retaining good computational properties. In December 2006, the OWL1.1 Member Submission [23] was made to the W3C. The W3C chartered the OWL Working Group as part of the Semantic Web Activity in September 2007. In April 2008, this group decided to call this new language OWL2, indicating a substantial revision. [24]

OWL 2 became a W3C recommendation in October 2009. OWL 2 introduces profiles to improve scalability in typical applications. [6]

### Acronym

Why not be inconsistent in at least one aspect of a language which is all about consistency?

Guus Schreiber, Why OWL and not WOL? [25]

OWL was chosen as an easily pronounced acronym that would yield good logos, suggest wisdom, and honor William A. Martin's One World Language knowledge representation project from the 1970s. [26] [27] [28]

A 2006 survey of ontologies available on the web collected 688 OWL ontologies. Of these, 199 were OWL Lite, 149 were OWL DL and 337 OWL Full (by syntax). They found that 19 ontologies had in excess of 2,000 classes, and that 6 had more than 10,000. The same survey collected 587 RDFS vocabularies. [29]

## Ontologies

An ontology is an explicit specification of a conceptualization.

Tom Gruber, A Translation Approach to Portable Ontology Specifications [30]

The data described by an ontology in the OWL family is interpreted as a set of "individuals" and a set of "property assertions" which relate these individuals to each other. An ontology consists of a set of axioms which place constraints on sets of individuals (called "classes") and the types of relationships permitted between them. These axioms provide semantics by allowing systems to infer additional information based on the data explicitly provided. A full introduction to the expressive power of the OWL is provided in the W3C's OWL Guide. [31]

OWL ontologies can import other ontologies, adding information from the imported ontology to the current ontology. [17]

### Example

An ontology describing families might include axioms stating that a "hasMother" property is only present between two individuals when "hasParent" is also present, and that individuals of class "HasTypeOBlood" are never related via "hasParent" to members of the "HasTypeABBlood" class. If it is stated that the individual Harriet is related via "hasMother" to the individual Sue, and that Harriet is a member of the "HasTypeOBlood" class, then it can be inferred that Sue is not a member of "HasTypeABBlood". This is, however, only true if the concepts of "Parent" and "Mother" only mean biological parent or mother and not social parent or mother.

## Species

### OWL sublanguages

The W3C-endorsed OWL specification includes the definition of three variants of OWL, with different levels of expressiveness. These are OWL Lite, OWL DL and OWL Full (ordered by increasing expressiveness). Each of these sublanguages is a syntactic extension of its simpler predecessor. The following set of relations hold. Their inverses do not.

• Every legal OWL Lite ontology is a legal OWL DL ontology.
• Every legal OWL DL ontology is a legal OWL Full ontology.
• Every valid OWL Lite conclusion is a valid OWL DL conclusion.
• Every valid OWL DL conclusion is a valid OWL Full conclusion.

#### OWL Lite

OWL Lite was originally intended to support those users primarily needing a classification hierarchy and simple constraints. For example, while it supports cardinality constraints, it only permits cardinality values of 0 or 1. It was hoped that it would be simpler to provide tool support for OWL Lite than its more expressive relatives, allowing quick migration path for systems using thesauri and other taxonomies. In practice, however, most of the expressiveness constraints placed on OWL Lite amount to little more than syntactic inconveniences: most of the constructs available in OWL DL can be built using complex combinations of OWL Lite features, and is equally expressive as the description logic ${\displaystyle {\mathcal {SHIF}}(\mathbf {D} )}$. [24] Development of OWL Lite tools has thus proven to be almost as difficult as development of tools for OWL DL, and OWL Lite is not widely used. [24]

#### OWL DL

OWL DL is designed to provide the maximum expressiveness possible while retaining computational completeness (either φ or ¬φ holds), decidability (there is an effective procedure to determine whether φ is derivable or not), and the availability of practical reasoning algorithms. OWL DL includes all OWL language constructs, but they can be used only under certain restrictions (for example, number restrictions may not be placed upon properties which are declared to be transitive; and while a class may be a subclass of many classes, a class cannot be an instance of another class). OWL DL is so named due to its correspondence with description logic, a field of research that has studied the logics that form the formal foundation of OWL.

#### OWL Full

OWL Full is based on a different semantics from OWL Lite or OWL DL, and was designed to preserve some compatibility with RDF Schema. For example, in OWL Full a class can be treated simultaneously as a collection of individuals and as an individual in its own right; this is not permitted in OWL DL. OWL Full allows an ontology to augment the meaning of the pre-defined (RDF or OWL) vocabulary. OWL Full is undecidable, so no reasoning software is able to perform complete reasoning for it.

### OWL2 profiles

In OWL 2, there are three sublanguages of the language. OWL 2 EL is a fragment that has polynomial time reasoning complexity; OWL 2 QL is designed to enable easier access and query to data stored in databases; OWL 2 RL is a rule subset of OWL 2.

## Syntax

The OWL family of languages supports a variety of syntaxes. It is useful to distinguish high level syntaxes aimed at specification from exchange syntaxes more suitable for general use.

### High level

These are close to the ontology structure of languages in the OWL family.

#### OWL abstract syntax

High level syntax is used to specify the OWL ontology structure and semantics. [32]

The OWL abstract syntax presents an ontology as a sequence of annotations, axioms and facts. Annotations carry machine and human oriented meta-data. Information about the classes, properties and individuals that compose the ontology is contained in axioms and facts only. Each class, property and individual is either anonymous or identified by an URI reference. Facts state data either about an individual or about a pair of individual identifiers (that the objects identified are distinct or the same). Axioms specify the characteristics of classes and properties. This style is similar to frame languages, and quite dissimilar to well known syntaxes for DLs and Resource Description Framework (RDF). [32]

Sean Bechhofer, et al. argue that though this syntax is hard to parse, it is quite concrete. They conclude that the name abstract syntax may be somewhat misleading. [33]

#### OWL2 functional syntax

This syntax closely follows the structure of an OWL2 ontology. It is used by OWL2 to specify semantics, mappings to exchange syntaxes and profiles. [34]

### Exchange syntaxes

 Filename extension .mw-parser-output .monospaced{font-family:monospace,monospace} .owx, .owl, .rdf application/owl+xml, application/rdf+xml [35] World Wide Web Consortium OWL 2 XML Serialization October 27, 2009;11 years ago, OWL Reference February 10, 2004;16 years ago Yes

#### RDF syntaxes

Syntactic mappings into RDF are specified [32] [36] for languages in the OWL family. Several RDF serialization formats have been devised. Each leads to a syntax for languages in the OWL family through this mapping. RDF/XML is normative. [32] [36]

#### OWL2 XML syntax

OWL2 specifies an XML serialization that closely models the structure of an OWL2 ontology. [37]

#### Manchester Syntax

The Manchester Syntax is a compact, human readable syntax with a style close to frame languages. Variations are available for OWL and OWL2. Not all OWL and OWL2 ontologies can be expressed in this syntax. [38]

### Examples

• The W3C OWL 2 Web Ontology Language provides syntax examples. [39]

#### Tea ontology

Consider an ontology for tea based on a Tea class. First, an ontology identifier is needed. Every OWL ontology must be identified by a URI (http://www.example.org/tea.owl, say). This example provides a sense of the syntax. To save space below, preambles and prefix definitions have been skipped.

OWL2 Functional Syntax
Ontology(<http://example.org/tea.owl>Declaration(Class(:Tea)))
OWL2 XML Syntax
<OntologyontologyIRI="http://example.org/tea.owl"...><Prefixname="owl"IRI="http://www.w3.org/2002/07/owl#"/><Declaration><ClassIRI="Tea"/></Declaration></Ontology>
Manchester Syntax
Ontology:<http://example.org/tea.owl>Class:Tea
RDF/XML syntax
<rdf:RDF...><owl:Ontologyrdf:about=""/><owl:Classrdf:about="#Tea"/></rdf:RDF>
RDF/Turtle
<http://example.org/tea.owl>rdf:typeowl:Ontology.:Teardf:typeowl:Class.

## Semantics

### Relation to description logics

OWL classes correspond to description logic (DL) concepts, OWL properties to DL roles, while individuals are called the same way in both the OWL and the DL terminology. [40]

In the beginning, IS-A was quite simple. Today, however, there are almost as many meanings for this inheritance link as there are knowledge-representation systems.

Ronald J. Brachman, What IS-A is and isn't [41]

Early attempts to build large ontologies were plagued by a lack of clear definitions. Members of the OWL family have model theoretic formal semantics, and so have strong logical foundations.

Description logics are a family of logics that are decidable fragments of first-order logic with attractive and well-understood computational properties. OWL DL and OWL Lite semantics are based on DLs. [42] They combine a syntax for describing and exchanging ontologies, and formal semantics that gives them meaning. For example, OWL DL corresponds to the ${\displaystyle {\mathcal {SHOIN}}^{\mathcal {(D)}}}$ description logic, while OWL 2 corresponds to the ${\displaystyle {\mathcal {SROIQ}}^{\mathcal {(D)}}}$ logic. [43] Sound, complete, terminating reasoners (i.e. systems which are guaranteed to derive every consequence of the knowledge in an ontology) exist for these DLs.

### Relation to RDFS

OWL Full is intended to be compatible with RDF Schema (RDFS), and to be capable of augmenting the meanings of existing Resource Description Framework (RDF) vocabulary. [44] A model theory describes the formal semantics for RDF. [45] This interpretation provides the meaning of RDF and RDFS vocabulary. So, the meaning of OWL Full ontologies are defined by extension of the RDFS meaning, and OWL Full is a semantic extension of RDF. [46]

### Open world assumption

[The closed] world assumption implies that everything we don’t know is false, while the open world assumption states that everything we don’t know is undefined.

Stefano Mazzocchi, Closed World vs. Open World: the First Semantic Web Battle [47]

The languages in the OWL family use the open world assumption. Under the open world assumption, if a statement cannot be proven to be true with current knowledge, we cannot draw the conclusion that the statement is false.

#### Contrast to other languages

A relational database consists of sets of tuples with the same attributes. SQL is a query and management language for relational databases. Prolog is a logical programming language. Both use the closed world assumption.

## Terminology

Languages in the OWL family are capable of creating classes, properties, defining instances and its operations.

### Instances

An instance is an object. It corresponds to a description logic individual.

### Classes

A class is a collection of objects. A class may contain individuals, instances of the class. A class may have any number of instances. An instance may belong to none, one or more classes.

A class may be a subclass of another, inheriting characteristics from its parent superclass. This corresponds to logical subsumption and DL concept inclusion notated ${\displaystyle \sqsubseteq }$.

All classes are subclasses of owl:Thing (DL top notated ${\displaystyle \top }$), the root class.

All classes are subclassed by owl:Nothing (DL bottom notated ${\displaystyle \bot }$), the empty class. No instances are members of owl:Nothing. Modelers use owl:Thing and owl:Nothing to assert facts about all or no instances. [48] [ self-published source ]

Class and their members can be defined in OWL either by extension or by intension. An individual can be explicitly assigned a class by a Class assertion, for example we can add a statement Queen elizabeth is a(n instance of) human, or by a class expression with ClassExpression statements every instance of the human class who has a female value to the sex property is an instance of the woman class.

#### Example

Let's call human the class of all humans in the world is a subclass of owl:thing. The class of all women (say woman) in the world is a subclass of human. Then we have ${\displaystyle {\textsf {owl:Nothing}}\sqsubseteq {\textsf {woman}}\sqsubseteq {\textsf {human}}\sqsubseteq {\textsf {owl:Thing}}}$

The membership of some individual to a class could be noted

 ClassAssertion( humanGeorge_Washington )

and class inclusion

 SubClassOf( womanhuman )

The first means "George Washington is a human" and the second "every woman is human".

### Properties

A property is a characteristic of a class - a directed binary relation that specifies some attribute which is true for instances of that class. Properties sometimes act as data values, or links to other instances. Properties may exhibit logical features, for example, by being transitive, symmetric, inverse and functional. Properties may also have domains and ranges.

#### Datatype properties

Datatype properties are relations between instances of classes and RDF literals or XML schema datatypes. For example, modelName (String datatype) is the property of Manufacturer class. They are formulated using owl:DatatypeProperty type.

#### Object properties

Object properties are relations between instances of two classes. For example, ownedBy may be an object type property of the Vehicle class and may have a range which is the class Person. They are formulated using owl:ObjectProperty.

### Operators

Languages in the OWL family support various operations on classes such as union, intersection and complement. They also allow class enumeration, cardinality, disjointness, and equivalence.

### Metaclasses

Metaclasses are classes of classes. They are allowed in OWL full or with a feature called class/instance punning.

## Public ontologies

### Browsers

The following tools include public ontology browsers:

## Limitations

• No direct language support for n-ary relationships. For example, modelers may wish to describe the qualities of a relation, to relate more than 2 individuals or to relate an individual to a list. This cannot be done within OWL. They may need to adopt a pattern instead which encodes the meaning outside the formal semantics. [57]

## Related Research Articles

The Semantic Web is an extension of the World Wide Web through standards set by the World Wide Web Consortium (W3C). The goal of the Semantic Web is to make Internet data machine-readable.

In computer science and information science, an ontology encompasses a representation, formal naming and definition of the categories, properties and relations between the concepts, data and entities that substantiate one, many, or all domains of discourse. More simply, an ontology is a way of showing the properties of a subject area and how they are related, by defining a set of concepts and categories that represent the subject.

The DARPA Agent Markup Language (DAML) was the name of a US funding program at the US Defense Advanced Research Projects Agency (DARPA) started in 1999 by then-Program Manager James Hendler, and later run by Murray Burke, Mark Greaves and Michael Pagels. The program focused on the creation of machine-readable representations for the Web.

The Resource Description Framework (RDF) is a family of World Wide Web Consortium (W3C) specifications originally designed as a metadata data model. It has come to be used as a general method for conceptual description or modeling of information that is implemented in web resources, using a variety of syntax notations and data serialization formats. It is also used in knowledge management applications.

OIL can be regarded as an ontology infrastructure for the Semantic Web. OIL is based on concepts developed in Description Logic (DL) and frame-based systems and is compatible with RDFS.

Description logics (DL) are a family of formal knowledge representation languages. Many DLs are more expressive than propositional logic but less expressive than first-order logic. In contrast to the latter, the core reasoning problems for DLs are (usually) decidable, and efficient decision procedures have been designed and implemented for these problems. There are general, spatial, temporal, spatiotemporal, and fuzzy description logics, and each description logic features a different balance between expressive power and reasoning complexity by supporting different sets of mathematical constructors.

RDF Schema is a set of classes with certain properties using the RDF extensible knowledge representation data model, providing basic elements for the description of ontologies. It uses various forms of RDF vocabularies, intended to structure RDF resources. RDF and RDFS can be saved in a triplestore, then one can entail some knowledge from them using a query language, like SPARQL.

Patrick John Hayes FAAAI is a British computer scientist who lives and works in the United States. As of March 2006, he is a Senior Research Scientist at the Institute for Human and Machine Cognition in Pensacola, Florida.

In computer science and artificial intelligence, ontology languages are formal languages used to construct ontologies. They allow the encoding of knowledge about specific domains and often include reasoning rules that support the processing of that knowledge. Ontology languages are usually declarative languages, are almost always generalizations of frame languages, and are commonly based on either first-order logic or on description logic.

RDFa is a W3C Recommendation that adds a set of attribute-level extensions to HTML, XHTML and various XML-based document types for embedding rich metadata within Web documents. The RDF data-model mapping enables its use for embedding RDF subject-predicate-object expressions within XHTML documents. It also enables the extraction of RDF model triples by compliant user agents.

The ultimate goal of semantic technology is to help machines understand data. To enable the encoding of semantics with the data, well-known technologies are RDF and OWL. These technologies formally represent the meaning involved in information. For example, ontology can describe concepts, relationships between things, and categories of things. These embedded semantics with the data offer significant advantages such as reasoning over data and dealing with heterogeneous data sources.

F-logic is a knowledge representation and ontology language. F-logic combines the advantages of conceptual modeling with object-oriented, frame-based languages and offers a declarative, compact and simple syntax, as well as the well-defined semantics of a logic-based language.

The Semantic Web Rule Language (SWRL) is a proposed language for the Semantic Web that can be used to express rules as well as logic, combining OWL DL or OWL Lite with a subset of the Rule Markup Language.

An RDF query language is a computer language, specifically a query language for databases, able to retrieve and manipulate data stored in Resource Description Framework (RDF) format.

Ian Robert Horrocks is a Professor of Computer Science at the University of Oxford in the UK and a Fellow of Oriel College, Oxford. His research focuses on knowledge representation and reasoning, particularly ontology languages, description logic and optimised tableaux decision procedures.

A semantic reasoner, reasoning engine, rules engine, or simply a reasoner, is a piece of software able to infer logical consequences from a set of asserted facts or axioms. The notion of a semantic reasoner generalizes that of an inference engine, by providing a richer set of mechanisms to work with. The inference rules are commonly specified by means of an ontology language, and often a description logic language. Many reasoners use first-order predicate logic to perform reasoning; inference commonly proceeds by forward chaining and backward chaining. There are also examples of probabilistic reasoners, including non-axiomatic reasoning systems, and probabilistic logic networks.

The Semantic Web Stack, also known as Semantic Web Cake or Semantic Web Layer Cake, illustrates the architecture of the Semantic Web.

WSML or Web Service Modeling Language is a formal language that provides a syntax and semantics for the Web Service Modeling Ontology (WSMO).

In computer science, information science and systems engineering, ontology engineering is a field which studies the methods and methodologies for building ontologies: formal representations of a set of concepts within a domain and the relationships between those concepts. In a broader sense, this field also includes a knowledge construction of the domain using formal ontology representations such as OWL/RDF. A large-scale representation of abstract concepts such as actions, time, physical objects and beliefs would be an example of ontological engineering. Ontology engineering is one of the areas of applied ontology, and can be seen as an application of philosophical ontology. Core ideas and objectives of ontology engineering are also central in conceptual modeling.

In the Semantic Web and in knowledge representation, a metaclass is a class whose instances are themselves classes. Similar to their role in programming languages, metaclasses in Semantic Web languages can have properties otherwise applicable only to individuals, while retaining the same class's ability to be classified in a concept hierarchy. This enables knowledge about instances of those metaclasses to be inferred by semantic reasoners using statements made in the metaclass. Metaclasses thus enhance the expressivity of knowledge representations in a way that can be intuitive for users. While classes are suitable to represent a population of individuals, metaclasses can, as one of their feature, be used to represent the conceptual dimension of an ontology. Metaclasses are supported in the ontology language OWL and the data-modeling vocabulary RDFS.

## References

1. Knublauch, Holger; Oberle, Daniel; Tetlow, Phil; Wallace, Evan (March 9, 2006). "A Semantic Web Primer for Object-Oriented Software Developers". W3C . Retrieved November 19, 2017.
2. "OWL 2 Web Ontology Language Document Overview (Second Edition)". W3C . 11 December 2012.
3. "OWL". W3.org. Retrieved 2017-02-23.
4. "Submission Request to W3C: OWL 1.1 Web Ontology Language". W3C. 2006-12-19.
5. "W3C Standard Facilitates Data Management and Integration". W3.org. 2009-10-27. Retrieved 15 October 2013.
6. Sirin, E.; Parsia, B.; Grau, B. C.; Kalyanpur, A.; Katz, Y. (2007). "Pellet: A practical OWL-DL reasoner" (PDF). Web Semantics: Science, Services and Agents on the World Wide Web. 5 (2): 51–53. doi:10.1016/j.websem.2007.03.004. Archived from the original (PDF) on 2007-06-27.
7. "RACER - Home". Racer-systems.com. Retrieved 2017-02-23.
8. Tsarkov, D.; Horrocks, I. (2006). "FaCT++ Description Logic Reasoner: System Description" (PDF). Automated Reasoning. Lecture Notes in Computer Science. 4130. pp. 292–297. CiteSeerX  . doi:10.1007/11814771_26. ISBN   978-3-540-37187-8.
10. "Home". HermiT Reasoner. Retrieved 2017-02-23.
11. Berners-Lee, Tim; James Hendler; Ora Lassila (May 17, 2001). "The Semantic Web A new form of Web content that is meaningful to computers will unleash a revolution of new possibilities". Scientific American. 284 (5): 34–43. doi:10.1038/scientificamerican0501-34. Archived from the original on April 24, 2013.
12. John Hebeler (April 13, 2009). . ISBN   978-0470418017.
13. Segaran, Toby; Evans, Colin; Taylor, Jamie (July 24, 2009). Programming the Semantic Web. O'Reilly Media. ISBN   978-0596153816.
14. Lacy, Lee W. (2005). "Chapter 10". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. ISBN   978-1-4120-3448-7.
15. Baader, Franz; Horrocks, Ian; Sattler, Ulrike (2005). "Description Logics as Ontology Languages for the Semantic Web". In Hutter, Dieter; Stephan, Werner (eds.). Mechanizing Mathematical Reasoning: Essays in Honor of Jörg H. Siekmann on the Occasion of His 60th Birthday. Heidelberg, DE: Springer Berlin. ISBN   978-3-540-25051-7.
16. Horrocks, Ian; Patel-Schneider, Peter F.; van Harmelen, Frank (2003). "From SHIQ and RDF to OWL: the making of a Web Ontology Language". Web Semantics: Science, Services and Agents on the World Wide Web. 1 (1): 7–26. CiteSeerX  . doi:10.1016/j.websem.2003.07.001.
17. World Wide Web Consortium (2010-02-06). "W3C Semantic Web Activity" . Retrieved 18 April 2010.
18. World Wide Web Consortium (2002-08-23). "Metadata Activity Statement". World Wide Web Consortium. Retrieved 20 April 2010.
19. World Wide Web Consortium (2002-08-23). "RDF Vocabulary Description Language 1.0: RDF Schema". RDF Vocabulary Description Language 1.0. World Wide Web Consortium. Retrieved 20 April 2010.
20. Lacy, Lee W. (2005). "Chapter 9 - RDFS". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. ISBN   978-1-4120-3448-7.
21. Patel-Schneider, Peter F.; Horrocks, Ian (2006-12-19). "OWL 1.1 Web Ontology Language". World Wide Web Consortium. Retrieved 26 April 2010.
22. Grau, B. C.; Horrocks, I.; Motik, B.; Parsia, B.; Patel-Schneider, P. F.; Sattler, U. (2008). "OWL 2: The next step for OWL" (PDF). Web Semantics: Science, Services and Agents on the World Wide Web. 6 (4): 309–322. doi:10.1016/j.websem.2008.05.001.
23. Herman, Ivan. "Why OWL and not WOL?". Tutorial on Semantic Web Technologies. World Wide Web Consortium. Retrieved 18 April 2010.
24. "Re: NAME: SWOL versus WOL". message sent to W3C webont-wg mailing list on 27 December 2001.
25. Ian Horrocks (2012). "Ontologe Reasoning: The Why and The How" (PDF). p. 7. Retrieved January 28, 2014.
26. "OWL: the original". July 7, 2003. Retrieved March 22, 2020.
27. Wang, T. D.; Parsia, B.; Hendler, J. (2006). "A Survey of the Web Ontology Landscape". The Semantic Web - ISWC 2006. Lecture Notes in Computer Science. 4273. p. 682. doi:10.1007/11926078_49. ISBN   978-3-540-49029-6.
28. Gruber, Tom (1993); "A Translation Approach to Portable Ontology Specifications", in Knowledge Acquisition, 5: 199-199
29. W3C (ed.). "OWL Web Ontology Language Guide".
30. Patel-Schneider, Peter F.; Horrocks, Ian; Patrick J., Hayes (2004-02-10). "OWL Web Ontology Language Semantics and Abstract Syntax". World Wide Web Consortium. Retrieved 18 April 2010.
31. Bechhofer, Sean; Patel-Schneider, Peter F.; Turi, Daniele (2003-12-10). "OWL Web Ontology Language Concrete Abstract Syntax". University of Manchester . Retrieved 18 April 2010.
32. Motik, Boris; Patel-Schneider, Peter F.; Parsia, Bijan (2009-10-27). "OWL 2 Web Ontology Language Structural Specification and Functional-Style Syntax". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
33. A. Swartz (September 2004). "application/rdf+xml Media Type Registration (RFC3870)". IETF. p. 2. Archived from the original on 2013-09-17. Retrieved 15 October 2013.
34. Patel-Schneider, Peter F.; Motik, Boris (2009-10-27). "OWL 2 Web Ontology Language Mapping to RDF Graphs". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
35. Motik, Boris; Parsia, Bijan; Patel-Schneider, Peter F. (2009-10-27). "OWL 2 Web Ontology Language XML Serialization". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
36. Horridge, Matthew; Patel-Schneider, Peter F. (2009-10-27). "OWL 2 Web Ontology Language Manchester Syntax". W3C OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
37. Hitzler, Pascal; Krötzsch, Markus; Parsia, Bijan; Patel-Schneider, Peter F.; Rudolph, Sebastian (2009-10-27). "OWL 2 Web Ontology Language Primer". OWL 2 Web Ontology Language. World Wide Web Consortium. Retrieved 15 October 2013.
38. Sikos, Leslie F. (2017). Description Logics in Multimedia Reasoning. Cham: Springer International Publishing. doi:10.1007/978-3-319-54066-5. ISBN   978-3-319-54066-5.
39. Brachman, Ronald J. (1983); What IS-A is and isn't: An analysis of taxonomic links in semantic networks, IEEE Computer, vol. 16, no. 10, pp. 30-36
40. Horrocks, Ian; Patel-Schneider, Peter F. "Reducing OWL Entailment to Description Logic Satisfiability" (PDF).
41. Hitzler, Pascal; Krötzsch, Markus; Rudolph, Sebastian (2009-08-25). Foundations of Semantic Web Technologies. CRCPress. ISBN   978-1-4200-9050-5.
42. McGuinness, Deborah; van Harmelen, Frank (2004-02-10). "OWL Web Ontology Language Overview". W3C Recommendation for OWL, the Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
43. Hayes, Patrick (2004-02-10). "RDF Semantics". Resource Description Framework. World Wide Web Consortium. Retrieved 18 April 2010.
44. Patel-Schneider, Peter F.; Hayes, Patrick; Horrocks, Ian (2004-02-10). "OWL Web Ontology Language Semantics and Abstract Syntax Section 5. RDF-Compatible Model-Theoretic Semantics". W3C Recommendation for OWL, the Web Ontology Language. World Wide Web Consortium. Retrieved 18 April 2010.
45. Mazzocchi, Stefano (2005-06-16). "Closed World vs. Open World: the First Semantic Web Battle". Archived from the original on 24 June 2009. Retrieved 27 April 2010.
46. Lacy, Lee W. (2005). "Chapter 12". OWL: Representing Information Using the Web Ontology Language. Victoria, BC: Trafford Publishing. ISBN   978-1-4120-3448-7.
47. OBO Technical WG. "The OBO Foundry". The OBO Foundry. Retrieved 2017-02-23.