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Document title:
OWL 2 Web Ontology Language
Overview and Rationale (Second Edition)
Peter F. Patel-Schneider, Bell Labs Research, Alcatel-Lucent
Ian Horrocks, Oxford University
Bernardo Cuenca Grau, Oxford University
OWL 1.1 extends the W3C OWL Web Ontology Language with a small but useful set of features that have been requested by users, for which effective reasoning algorithms are now available, and that OWL tool developers are willing to support. The new features include extra syntactic sugar, additional property and qualified cardinality constructors, extended datatype support, simple metamodelling, and extended annotations. This document provides a high-level overview of these features.
Status of this Document
This document is an evolution of the OWL 1.1 Web Ontology Language: Overview document that forms part of the OWL 1.1 Web Ontology Language W3C Member Submission. Comments are welcome. Please send feedback to public-owl-wg@w3.org, which has a public archive. Bug reports can be directed there.

1 Overview and Rationale

The initial design of the W3C OWL Web Ontology Language (OWL) was conservative in several ways. Constructs that did not have considerable support from within the W3C Web Ontology Working Group were not included. Constructs for which effective reasoning methods were not known or expected to be known in future were also not included.

Usage of OWL has identified several constructs that are of considerable utility and that fit well within the representation philosophy. Advances in the theory of Description Logics have provided a basis for reasoning with constructs that are not part of OWL, or not part of OWL DL.

For both these reasons, it was decided at the first OWL: Experiences and Directions workshop to design an extension to OWL. The extension is designed to provide simple extensions to OWL that

  1. have been requested by major users of OWL;
  2. have effective reasoning methods, as evidenced by theoretical results; and
  3. are expected to be implemented by the developers of OWL reasoners.

This document provides a high-level overview of OWL 1.1. The OWL 1.1 syntax document [OWL 1.1 Syntax] defines the normal syntax for OWL 1.1 and give some informal information on the semantics of the new constructs. The OWL 1.1 semantics document [OWL 1.1 Semantics] formally defines the semantics of all of OWL 1.1 which is essentially the same as that of [SROIQ] with a simple datatype theory.

2 Features

The features added in OWL 1.1 fall into four main categories:

  1. syntactic sugar to make some commonly-stated things easier to say;
  2. new Description Logic constructs;
  3. expanded datatype expressiveness; and
  4. metamodeling constructs.

2.1 Syntactic Sugar

OWL 1.1 provides two constructs that are simply syntactic sugar, to make some common idioms easier to write.

The first syntactic sugar construct, DisjointUnion, defines one description as the disjoint union of a set of descriptions. It is simply a combination of a DisjointClasses axiom and an EquivalentClasses axiom of the first description as the union of the rest.

The second syntactic sugar construct is negative property membership assertions NegativeObjectPropertyAssertion and NegativeDataPropertyAssertion.


OWL 1.1 provides extra Description Logic expressive power, moving from the SHOIN Description Logic that underlies OWL DL to the SROIQ Description Logic. The additions are:

  1. qualified cardinality restrictions such as ObjectMinCardinality(2 friendOf hacker);
  2. local reflexivity restrictions on noncomplex properties only, such as ObjectExistsSelf(likes) for narcissists;
  3. reflexive, irreflexive, symmetric, and asymmetric properties for noncomplex properties only, such as ReflexiveObjectProperty(knows) and IrreflexiveObjectProperty(husbandOf);
  4. disjoint properties for non-complex properties only, such as DisjointObjectProperties(childOf spouseOf); and
  5. property chain inclusion axioms such as SubObjectPropertyOf(SubObjectPropertyChain(owns part) owns), provided that there are no cyclic inclusions.

2.3 Datatypes

OWL 1.1 allows user-defined datatypes, using a mechanism similar to that in Protege [Protege]. User-defined datatypes can use most of the well-behaved datatype facets available from [XML Schema Datatypes].

This facility can be used in defining new datatypes that can be used in the ontology, as in SubClassOf(Adult DataSomeValuesFrom(age DatatypeRestriction(xsd:integer minInclusive "18"^^xsd:integer)).

Simple relationships between values of functional data-valued properties can be used as restrictions, as in DataSomeValuesFrom(shoeSize IQ greaterThan). The underlying theory of datatypes used in these restrictions is taken from [XML Schema Datatypes].

2.4 Metamodeling and annotations

In OWL 1.1 a name (such as Person) can be used as any or all of an individual, a class, and a property. The computational problems that would arise if this were treated as in RDF are avoided by ensuring that no aspect of the use of the name as an individual has any effect on the meaning of the name as a class. Such a treatment of metamodeling is often called punning.

3 References

[OWL 1.1 Specification]
OWL 1.1 Web Ontology Language: Structural Specification and Functional-Style Syntax Peter F. Patel-Schneider, Ian Horrocks, and Boris Motik, 2007.
[OWL 1.1 Semantics]
OWL 1.1 Web Ontology Language: Model-Theoretic Semantics. Bernardo Cuenca Grau and Boris Motik, 2007.
The Even More Irresistible SROIQ Ian Horrocks, Oliver Kutz, and Uli Sattler. In Proc. of the 10th Int. Conf. on Principles of Knowledge Representation and Reasoning (KR 2006). AAAI Press, 2006.
[XML Schema Datatypes]
XML Schema Part 2: Datatypes Second Edition. Paul V. Biron and Ashok Malhotra, eds. W3C Recommendation 28 October 2004.
Protege web page. http://protege.stanford.edu/.