OWL (the Web Ontology Language)
is being designed by the W3C Web Ontology Working Group to provide a
language that can be used for applications that need to understand the content
of information instead of just understanding the human-readable presentation of
content. OWL facilitates greater machine readability of web content than
that supported by XML,
RDF, and RDF-S by providing additional vocabulary for term
The OWL language provides three increasingly expressive
sublanguages: OWL Lite, OWL DL, and OWL Full.
This document provides an introduction to OWL by first introducing the
simplest language -- OWL Lite.
OWL DL and OWL Full include the same complete OWL vocabulary
however OWL DL is subject to some constraints discussed in
OWL Species discussion in the
Since OWL DL and OWL Full include the same vocabulary, they
are handled together in this document.
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
Recommendations and other technical reports is available at http://www.w3.org/TR/.
This document is a working document for the use by W3C Members and other
interested parties. It may be updated, replaced or made obsolete by other
documents at any time.
This document has been produced by the Web Ontology Working Group, as part
of the W3C Semantic Web
Activity. The goals of the Web Ontology working group are discussed in the
Web Ontology Working Group
Comments on this document should be sent to the W3C mailing list firstname.lastname@example.org
There are no patent
disclosures related to this work at the time of this writing.
- OWL Lite
RDF Schema Features Synopsis
- OWL Lite
Equality and Inequality Synopsis
- OWL Lite
Property Characteristics Synopsis
- OWL Lite
Restricted Cardinality Synopsis
- OWL Lite
- OWL Lite
Header Information Synopsis
OWL DL and OWL Full Synopsis
DL and OWL Full Class Axioms Synopsis
DL and OWL Full Boolean Combinations of Class Expressions Synopsis
DL and OWL Full Arbitrary Cardinality Synopsis
DL and OWL Full Filler Information Synopsis
Description of OWL Lite
Lite RDF Schema Features
Lite Equality and Inequality
Lite Property Characteristics
Lite Property Type Restrictions
Lite Restricted Cardinality
Lite Header Information
Language Description of OWL DL and OWL Full
This document describes OWL (the Web Ontology Language)
that is being designed by the W3C
Web Ontology Working Group
to provide a language that can be used for applications that need to understand
the content of information, instead of presenting just presenting
human-readable content. OWL can be used to explicitly
represent term vocabularies and the relationships between entities in
these vocabularies. This representation of terms and their interrelationships
creates an ontology.
The ontology language in OWL is more expressive than that in
XML, RDF, and RDF-S, and thus OWL goes beyond these language in
its ability to represent machine readable content on the web.
OWL is a
revision of the DAML+OIL web
ontology language incorporating lessons learned from the design and
application of DAML+OIL.
The goal of this document is to provide a simple introduction to OWL by
providing a language feature listing with very brief feature descriptions.
For a more complete description of OWL,
see the OWL Reference,
and the OWL
Abstract Syntax and Semantics documents.
This document begins by describing a subset of the language, called OWL
Lite. The goal of OWL Lite is to provide a language
that is a simpler for tool builders to support than the full OWL language.
One expectation is that tools will facilitate widespread adoption of OWL
and thus the OWL language
designers should attempt to create a language to which tool developers
While it is widely appreciated that all of the features in languages such
DAML+OIL are important to some users, it is also understood that languages as
expressive as DAML+OIL may be daunting to some groups who are trying to support
a tool suite for the entire language. In order to provide a language that is
approachable to a wider audience, OWL Lite has been defined a subset of OWL.
OWL Lite attempts to capture many of the commonly used
features of OWL and DAML+OIL. It also attempts to describe a useful language
that provides more than RDF-S meeting the goal of adding functionality that is
important to support web applications.
There are two interpretations for the full OWL vocabulary - one used in
OWL DL that is more restricted and one used for OWL Full that is less restrictive.
Since the vocabularies for OWL DL and OWL Full are identical, this document
only distinguishes between the OWL Lite vocabulary and the full OWL vocabulary.
For more on the motivation for OWL Lite, OWL DL, and OWL Full,
For more information about the interpretations for OWL DL and OWL Full,
OWL Abstract Syntax and Semantics Document.
2. Language SynopsisThis section contains the
language synopsis for OWL Lite, OWL DL, and OWL Full.
In this document, italicized terms are terms in OWL.
Capitalization in OWL terms in this document is consistent with
capitalization in the reference language document.
Prefixes of rdf: or rdfs: are used when terms are in the RDF or
RDF-S namespaces. Otherwise terms are in the OWL namespace.
2.1 OWL Lite Synopsis
The list of OWL Lite language constructs is given below.
2.1.1 OWL Lite RDF Schema Features
2.1.2 OWL Lite Equality and Inequality
2.1.3 OWL Lite Property Characteristics
- FunctionalProperty (unique)
- InverseFunctionalProperty (unambiguous)
- allValuesFrom (universal local range restrictions; previously
- someValuesFrom (existential local range restrictions; previously
2.1.4 OWL Lite Restricted Cardinality
- minCardinality (restricted to 0 or 1)
- maxCardinality (restricted to 0 or 1)
- cardinality (restricted to 0 or 1)
2.1.5 OWL Lite Datatypes Synopsis
Following the decisions of RDF Core.
2.1.6 OWL Lite Header Information Synopsis
2.2 OWL DL and Full Synopsis
The list of OWL DL and OWL Full language constructs that are in addition to
those of OWL Lite are given below.
2.2.1 OWL Class Axioms Synopsis
- oneOf (enumerated classes)
- sameClassAs applied to class expressions
- rdfs:subClassOf applied to class expressions
2.2.2 OWL Boolean Combinations of Class
2.2.3 OWL Arbitrary Cardinality Synopsis
2.2.4 OWL Filler Information Synopsis
- hasValue Descriptions can include specific value information
3. Language Description of OWL Lite
This section provides an expanded description of
the OWL Lite language features in English. An abstract syntax is used
for presentation of the language. OWL Lite has a subset of
the full OWL language
constructors and has a few limitations. Unlike the full OWL language (and
DAML+OIL), classes can only be defined in terms of named superclasses and only
certain kinds of restrictions can be used.
Equivalence between classes and subclass relationships between classes
are only allowed to be stated on named classes.
Similarly, property restrictions in OWL-Lite use only named classes.
OWL Lite also has a limited notion of cardinality -
the only cardinalities allowed to be explicitly stated are 0 or 1.
3.1 OWL Lite RDF Schema Features
OWL can be viewed as an extension of a restricted view of the RDF language.
Therefore every OWL document is an RDF document, but not all RDF documents are OWL
documents. All terms are in the OWL namespace unless explicitly stated
otherwise. Thus, the term Class is more precisely stated as
owl:Class and rdfs:subPropertyOf indicates that
subProperty is from
the rdfs namespace. This document uses the term "individual"
to objects that belong to classes (e.g., the individual Deborah belongs to the
class Person) as well as to objects that are datatypes (e.g., the individual 4
is an integer).
The following OWL Lite features related to RDF Schema are included.
- Class: Classes can be made to be equivalent to or subclasses
of intersections of other classes. There is a built-in most
general class named Thing that is the class of all individuals and the
superclass of all classes. We may choose to make a new subclass of the class
Thing named Mammal. (Later we may choose to refine Mammal to include
additional information.) We may also create a new class named Person that is a
subclass of Mammal. From this a reasoner can deduce that any instance of the
class Person is also an instance of the class Mammal. Note that there is no
limitation on cycle creation in subclass hierarchies.
- rdfs:Property: A term that will be used as a relationship
between individuals is a property. Examples of properties include:
hasChild, hasRelative, hasSibling, and hasAge. The first three
relate an instance of a class Person to another instance of the
class Person and the last one (hasAge) relates an instance
of the class Person to an instance of the datatype Integer.
- rdfs:subClassOf: Class hierarchies may be created by stating
that classes are subclasses of other classes. For example, the class Person
could be stated to be a subclass of the class Mammal.
From this a reasoner can
deduce that if X is a Person, then X is a Mammal.
- rdfs:subPropertyOf: Property hierarchies may be created by
stating that some properties are subproperties of other properties. For
example, hasSibling may be stated to be a subproperty of hasRelative. From
this a reasoner can deduce that if X is related to Y by the hasSibling
property, then X is also related to Y by the hasRelative property.
- rdfs:domain: Properties may be stated to have domains,
(i.e., if X is related to Y by a property p with a domain class Z,
then X must
be an instance of the domain class Z). For example, the property hasChild may be
stated to have the domain of Mammal. From this a reasoner can deduce that if X
is related to Y by the hasChild property, i.e., Y is the child of X, then X is
a Mammal. Note that these are called global restrictions since the restriction
is stated on the property and not just on the property when it is associated
with a particular class. See the discussion below on local restrictions for
- rdfs:range: Properties may be stated to have ranges, (i.e.,
if X is related to Y by a property p with a range class Z,
then Y must be an
instance of the range class Z). For example, the property hasChild may be stated
to have the range of Mammal. From this a reasoner can deduce that if Louise is
related to Deborah by the hasChild property, i.e., Deborah is the child of
Louise, then Deborah is a Mammal. Range is also a global restriction as is
domain above. See the discussion below on local restrictions for more
- Individual: Individuals may be described as an instance of a
and properties may also be used to relate one individual to another. For
example, an individual named Deborah may be described as an instance of the
class Person and the property hasEmployer may be used to relate the individual
Deborah to the individual StanfordUniversity.
3.2 OWL Lite Equality and Inequality
The following OWL Lite features related to equality or inequality are included.
- sameClassAs: Two classes may be stated to be the same (i.e.,
they may be stated to be different names for the same set of individuals).
Equality can be used to create synonymous classes. For example, Car can
be stated to be sameClassAs Automobile. From this a reasoner can deduce
that any individual that is an instance of Car is also an instance of
Automobile and vice versa.
- samePropertyAs: Two properties may be stated to be the same.
Equality may be used to create synonymous properties.
For example, hasLeader may
be stated to be the samePropertyAs hasHead. From this a reasoner can
deduce that if X is related to Y by the property hasLeader, X is also related
to Y by the property hasHead and vice versa. A reasoner can also deduce that
hasLeader is a subproperty of hasHead and hasHead is a subProperty of
- sameIndividualAs: Two individuals may be stated to be the
same. Equality may be used to create a number of different names that
refer to the same individual. For example, the individual
Deborah may be stated to be the same individual as DeborahMcGuinness.
- differentIndividualFrom: Two individuals may be stated to be
different from each other. For example, the individuals Frank and Deborah may
be stated to be different from each other. From this inequality,
a reasoner can deduce
that Frank and Deborah refer to two unique individuals. Thus, if the
individuals Frank and Deborah are both values for a property that is stated to
be functional (thus the property has at most one value), then there is a
contradiction. Stating differences can be important in systems such as OWL
(and RDF) that do not assume that individuals have one and only one name. For
example, with no additional information, a reasoner will not
deduce that Frank and Deborah refer to distinct individuals.
3.3 OWL Lite Property CharacteristicsThere are
special identifiers in OWL Lite that are used to provide information
concerning properties and their values.
- inverseOf: One property may be stated to be the inverse of
another property. If the property P1 is stated to be the inverse
of the property P2, then if X is related to Y by the P2 property, then Y is
related to X by the P1 property. For example, if hasChild is the inverse of
hasParent and Deborah hasParent Louise, then a reasoner can deduce that Louise
- TransitiveProperty: Properties may be stated to be
transitive. If a property is transitive, then if the pair (x,y) is an
of the transitive property P, and the pair (y,z) is an instance of P, then the
pair (x,z) is also an instance of P. For example, if ancestor is stated to be
transitive, and if Sara is an ancestor of Louise (i.e., (Sara,Louise) is an
instance of the property ancestor)
and Louise is an ancestor of Deborah (i.e.,
(Louise,Deborah) is an instance of the property ancestor), then a reasoner can deduce that
Sara is an ancestor of Deborah (i.e., (Sara,Deborah) is an instance of the
The same OWL DL and DAML+OIL side conditions hold that restrict transitive
properties (and their superproperties) from having an atmost1 or an exactly1
restriction. See the property axiom section of the abstract syntax document
for more information.
- SymmetricProperty: Properties may be stated to be symmetric.
If a property is symmetric, then if the pair (x,y) is an instance of the
symmetric property P, then the pair (y,x) is also an instance of P. For
example, friend may be stated to be a symmetric property. Then a reasoner that
is given that Frank is a friend of Deborah can deduce that Deborah is a
friend of Frank. Note that properties that are to be made symmetric
may not have arbitrary domains and ranges.
- FunctionalProperty : Properties may be stated to have a
If a property is a FunctionalProperty, then it has no more than one
value for each individual and it may have no values for an individual.
This characteristic has been referred to
as having a unique property.
Alternatively, the property's minimum
cardinality is zero and its maximum cardinality is 1. For example,
hasPrimaryEmployer may be stated to be a FunctionalProperty.
From this a reasoner may deduce that
no individual may have more than one primary employer.
This does not imply that every Person must
have at least one primary employer however. OWL Lite includes the same
side condition as is stated in the DAML+OIL specification and
OWL DL that does not allow
transitive properties nor any of their superproperties to be declared
functional. For more information on the details of the limitation, see the
Warning under the property element section of the DAML+OIL reference
description or in a research
paper by Horrocks, Sattler, and Tobies showing the undecidability that
would follow from violating this restriction.
See the semantics document for more information concerning the restrictions.
Properties may be
stated to be inverse functional. If a property is inverse functional then the
inverse of the property is functional. Thus the inverse of the property has at
most one value for each individual.
This characteristic has also been referred to as
an unambiguous property. For
example, hasUSSocialSecurityNumber (a unique identifier for United States
residents) may be stated to be inverse functional (or unambiguous). The
inverse of this property (which may be referred to as isTheSocialSecurityNumberFor) has
at most one value for an individual.
Thus any one person's social security number is the only
value for their isTheSocialSecurityNumberfor property.
From this a reasoner can
deduce that no two different individual instances of Person have the identical
US Social Security Number. Also, a reasoner can deduce that if two instances
of Person have the same social security number, then those two instances refer
to the same individual. See the Reference Document for more
3.4 OWL Lite Property Type Restriction
OWL Lite allows restrictions to be placed on the type
of values for a property.
The following two restrictions are placed on properties with respect to a class
and thus have the impact of limiting the extent of the class with the value
- allValuesFrom (toClass in DAML+OIL):
The restriction allValuesFrom is stated
on a property with respect to a class. A property on a particular class may
have a local range restriction associated with it. Thus if an
individual instance of the class is related by the property to a second
individual, then the second individual can be inferred to be an instance of
the local range restriction class. For example, the class Person may have a
property called hasOffspring restricted to have allValuesFrom the class
Person. This means that if an individual person Louise is related by the
property hasOffspring to the individual Deborah, then from this a reasoner can
deduce that Deborah is an instance of the class Person. This
restriction allows the
property hasOffspring to be used with other classes, such as the class Cat,
and have an appropriate value restriction associated with the use of the
property on that class. In this case, hasOffspring would have the local range
restriction of Cat when associated with the class Cat and would have the local
range restriction Person when associated with the class Person. Note that a
reasoner can not deduce from an allValuesFrom restriction alone that there is
at least one value for the property.
- someValuesFrom: (hasClass in DAML+OIL):
The restriction someValuesFrom is stated on a property with respect to a class. A
particular class may have a restriction on a property that at least one value
for that property is of a certain type. For example, the class
SemanticWebPaper may have a someValuesFrom restriction on the
hasKeyword property that states that some value for the hasKeyword
property should be an instance of the class SemanticWebTopic. This allows for
the option of having multiple keywords and as long as one or more is an
instance of the class SemanticWebTopic, then the paper would be consistent
with the someValuesFrom restriction. Unlike allValuesFrom,
someValuesFrom does not restrict all the values of the property to be
instances of the same class. If myPaper is an individual instance of the
SemanticWebPaper class, then myPaper is related by the hasKeyword
property to at least one individual instance of the SemanticWebTopic class.
Note that a reasoner can not deduce (as it could with allValuesFrom
restrictions) that all values of hasKeyword are instances of the
3.5 OWL Lite Restricted Cardinality
A limited form of cardinality restrictions have been included in OWL Lite.
OWL (and OWL Lite) cardinality restrictions are referred to as
local restrictions, since they
are stated on properties with respect to a particular class. That is, the
restrictions limit the cardinality of that property on instances of the class.
OWL Lite cardinality restrictions are limited because they only allow statements
concerning cardinalities of value 0 or 1 (they do not allow arbitrary values
for cardinality, as is the case in full OWL).
Alternate namings for these restricted forms of cardinality were discussed.
Current recommendations are to include any such names in a front end system.
More on this topic is available on the publically available webont mail
archives with the most relevant message at
- minCardinality: Cardinality is stated on a property with
respect to a particular class. If a minCardinality of 1 is stated on a
property with respect to a class, then any instance of that class will be
related to at least one individual by that property. This restriction
is another way of
saying that the property is required for all individual instances of
the class. For example, the class Person would not have any minimum cardinality
restrictions stated on a hasOffspring property since not all person have offspring. The class Parent, however would have a minimum cardinality of 1
on the hasOffspring property. If a reasoner knows that Louise is a Person, then nothing can be deduced about a minimum cardinality for her hasOffspring property. Once it is discovered that Louise is an instance of Parent, then a
reasoner can deduce that Louise is related to at
least one individual by the hasOffspring property. From this information
alone, a reasoner can not deduce any maximum number of offspring for
individual instances of the class parent. In OWL Lite the only minimum
cardinalities allowed are 0 or 1. A minimum cardinality of zero on a property
just states (in the absence of any more specific information) that the
property is optional with respect to a class. For example, the property has
Offspring may have a minimum cardinality of zero on the class Person (while it
is stated to have the more specific information of minimum cardinality of one
on the class Parent).
- maxCardinality: Cardinality is stated on a property with
respect to a particular class. If a maxCardinality of 1 is stated on a
property with respect to a class, then any instance of that class will be
related to at most one individual by that property. A maxCardinality 1 restriction is
sometimes called a functional or unique property. For example, the property
hasRegisteredVotingState on the class UnitedStatesCitizens may have a maximum
cardinality of one (because people are only allowed to vote in only one
state). From this a reasoner can deduce that individual instances of the class
USCitizens may not be related to two or more distinct individuals through the
hasRegisteredVotingState property. From a maximum cardinality one restriction
alone, a reasoner can not deduce a minimum cardinality of 1. It may be useful
to state that certain classes have no values for a particular property. For
example, instances of the class UnmarriedPerson should not be related to
any individuals by the property hasSpouse. This situation is
represented by a maximum cardinality of zero on the hasSpouse property on
the class UnmarriedPerson.
- cardinality: Cardinality is provided as a convenience when
it is useful to state that a property on a class has both
minCardinality 0 and maxCardinality 0 or both
minCardinality 1 and maxCardinality 1. For example, the class
Person has exactly one value for the property hasBirthMother. From this a
reasoner can deduce that no two distinct individual instances of the class
Mother may be values for
the hasBirthMother property of the same person.
3.6 OWL Lite Datatypes
Datatypes will be included OWL Lite. Thus, for
example, a range could be stated to be XSD:decimal. The exact details
of OWL datatypes are
dependent upon the RDF Core Group's decisions on datatypes for RDF. See
datatypeProperty and objectTypeProperty in the Reference specification for
3.7 OWL Lite Header Information
OWL supports standard notions of ontology referencing, inclusion, and
meta-information. All three levels of OWL include ways of specifying
ontologies to import,
ontology version information,
prior ontology version information,
ontologies known to be backward compatible,
and ontologies known to be incompatible.
The reference document includes information in its ontology elements section
describing these notions.
4. Incremental Language Description of OWL DL and OWL FULL
Both OWL DL and OWL Full use the same vocabulary although OWL DL is subject
to some restrictions. The semantics document explains the distinctions
and limitations. We describe the OWL DL and OWL Full vocabulary that
extends the constructions of
OWL Lite with the following.
This document provides a high level
description of OWL by providing a feature synopsis of OWL Lite
and the full OWL vocabulary used in both OWL DL and OWL Full.
It provides simple English descriptions of the
constructs along with simple examples. It makes no attempt to include a
It references the OWL reference
OWL Guide, and the
OWL Abstract Syntax and Semantics document for more details.
Previous versions (July 29, 2002,
8, 2002, June
23, 2002, May
26, 2002, and May
15, 2002) of this document provide the historical view of the evolution of
OWL Lite and the issues discussed in its evolution.
- oneOf (enumerated classes): Classes can be described by
enumeration of the individuals that make up the class. The members of the
class are exactly the set of enumerated individuals; no more, no less. For
example, the class of daysOfTheWeek can be described by simply enumerating the
individuals Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday.
From this a reasoner can deduce the maximum cardinality (7) of any property
that has daysOfTheWeek as its allValuesFrom restriction.
- hasValue (property values): A property can be required to
have a certain individual as a value (also sometimes referred to as property
values). For example, instances of the class of dutchCitizens can be
characterized as those people that have theNetherlands as a value of their
nationality. (TheNetherlands itself is an instance of the class of
- disjointWith: Full OWL allows the statement
that classes are disjoint.
For example, in OWL Man and Woman can be stated to be disjoint
classes. From this disjointWith statement, a reasoner can deduce
an inconsistency when an individual is stated to be an instance of
both and similarly a reasoner
can deduce that if A is an instance of Man, then A is
not an instance of Woman.
- unionOf, complementOf, and intersectionOf (Boolean
combinations): OWL allows arbitrary Boolean combinations of classes:
IntersectionOf, UnionOf, and complementOf. For example, taking the
intersection of the class of DutchCitizens with the class of SeniorCitizens
describes the class of DutchSeniorCitizens.
Using complement, we could state
that Children are not SeniorCitizens (i.e. the class Children is a subclass
of the complement of SeniorCitizens). Citizenship of the European Union could
be described as the union of the citizenship of all member states.
- minCardinality, maxCardinality, cardinality (full
cardinality): While in OWL Lite, cardinalities are restricted to at least, at
most or exactly 1 or 0, full OWL allows cardinality statements for arbitrary
non-negative integers. For example the class of DINKs ("Dual Income, No Kids")
would restrict the cardinality of the property hasIncome to a minimum
cardinality of two (while the property hasChild would have be restricted to
- complex classes : In many constructs,
OWL Lite restricts the syntax to
single class names (e.g. in subClassOf or equivalentClass statements). Full
OWL extends this restriction to allow arbitrarily complex class
descriptions, consisting of enumerated classes, property restrictions, and
Boolean combinations. OWL also includes a special "bottom" class with
the name Nothing that is the empty class.
Also, OWL full allows classes to be used as instances (and OWL DL and OWL Lite do
not). For more on this topic, see the "Design for Use" section of the Guide document.