Reference description of the
DAML+OIL (March 2001) ontology markup language

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DAML+OIL (March 2001) version (revision 4.2): Frank van Harmelen, Peter F. Patel-Schneider and Ian Horrocks, editors.

Contributors: Tim Berners-Lee, Dan Brickley, Dan Connolly, Mike Dean, Stefan Decker, Pat Hayes, Jeff Heflin, Jim Hendler, Ora Lassila, Deb McGuinness, Lynn Andrea Stein, ...


DAML+OIL is a semantic markup language for Web resources. It builds on earlier W3C standards such as RDF and RDF Schema, and extends these languages with richer modelling primitives. DAML+OIL provides modelling primitives commonly found in frame-based languages. DAML+OIL (March 2001) extends DAML+OIL (December 2000) with values from XML Schema datatypes. The language has a clean and well defined semantics.

This document gives a systematic, compact and informal description of all the modelling primitives of DAML+OIL (March 2001). We expect this document to serve as a reference guide for users of the DAML+OIL language.


Introductory remarks

A DAML+OIL knowledge base is a collection of RDF triples. DAML+OIL prescribes a specific meaning for triples that use the DAML+OIL vocabulary. This document informally specifies which collections of RDF triples constitute the DAML+OIL vocabulary and what the prescribed meaning of such triples is.

Different syntactic forms

As with any set of RDF triples, DAML+OIL triples can be represented in many different syntactic forms (as described in the RDF specification). The current document uses a specific RDF syntactic form for these triples. However, it is also allowed to use any other syntactic RDF form that results in the same underlying set of RDF triples as the syntax used in this document. Such other syntactic form would then carry exactly the same prescribed meaning as the equivalent syntactic form used in this document. See Syntax Note for an example of this.

Mixing DAML+OIL with arbitrary RDF

As stated above, DAML+OIL assigns a specific meaning to certain RDF triples. The model-theoretic semantics specifies exactly which triples are assigned a specific meaning, and what this meaning is. DAML+OIL only provides a semantic interpretation for those parts of an RDF graph that instantiate the schema defined in daml+oil.daml. Any additional RDF statements, resulting in additional RDF triples are perfectly allowed, but DAML+OIL is silent on the semantic consequences (or lack thereof) of such additional triples. See Mixing Note for an example of this. The KIF axiomatization provides a meaning for all RDF triples, but non DAML+OIL triples are only modelled as triples, nothing deeper.

Language structure

A DAML+OIL ontology is made up of several components, some of which are optional, and some of which may be repeated. See the index for a listing of all these components. Througout this document, DAML+OIL constructs will be presented in a structured format, and not as bare RDF triples. This structured RDF format is more natural to read, but, of course, any way of generating the same RDF triples as generated by the structured RDF format is equivalent.

A DAML+OIL ontology consists of zero or more headers, followed by zero or more class elements, property elements, and instances.


An daml:Ontology element contains zero or more version information and imports elements.

<Ontology rdf:about="">
  <versionInfo>$Id: reference.html,v 1.10 2001/04/11 16:27:53 mdean Exp $</versionInfo>
  <rdfs:comment>An example ontology</rdfs:comment>
  <imports rdf:resource=""/>

Version information

The daml:versionInfo element generally contains a string giving information about this version, for example RCS/CVS keywords. This element does not contribute to the logical meaning of the ontology. See the example above.


Each daml:imports statement references another DAML+OIL ontology containing definitions that apply to the current DAML+OIL resource. Each reference consists of a URI specifying from where the ontology is to be imported from. See the example above. Imports statements are transitive, that is, if ontology A imports B, and B imports C, then A imports both B and C. Importing an ontology into itself is considered a null action, so if ontology A imports B and B imports A, then they are considered to be equivalent.

Note that namespaces only provide a mechanism for creating unique names for elements, and do not actually include definitions in the way that imports does. Similarly, imports statements do not set up a shorthand notation for names. Therefore, it is common to have imports statements that correspond to each namespace. However, additional imports may be used for ontologies that provide definitions without introducing any new names.

Objects and Datatype Values

DAML+OIL divides the universe into two disjoint parts. One part consists of the values that belong to XML Schema datatypes. This part is called the datatype domain. The other part consists of (individual) objects that are considered to be members of classes described within DAML+OIL (or RDF). This part is called the object domain.

DAML+OIL is mostly concerned with the creation of classes that describe (or define) part of the object domain. Such classes are called object classes and are elements of daml:Class, a subclass of rdfs:Class. DAML+OIL (March 2001) also allows the use of XML Schema datatypes to describe (or define) part of the datatype domain. These datatypes are used within DAML+OIL simply by including their URIs within a DAML+OIL ontology. They are (implicitly) elements of daml:Datatype. Datatypes are not DAML+OIL individual objects.

Class elements

A class element, daml:Class, contains (part of) the definition of an object class. A class element refers to a class name (a URI), (we will refer to this class as C) and contains

Notice that the first two elements state necessary but not sufficient conditions for class membership. The final four elements state both necessary and sufficient conditions.

Class expressions

A class expression is the name used in this document for either

Each class expression either refers to a named class, namely the class that is identified by the URI, or implicitly defines an anonymous class, respectively the class that contains exactly the enumerated elements, or the class of all instances which satisfy the property-restriction, or the class that satisfies the boolean combination of such expressions.

Two class names are already predefined, namely the classes daml:Thing and daml:Nothing. Every object is a member of daml:Thing, and no object is a member daml:Nothing. Consequently, every class is a subclass of daml:Thing and daml:Nothing is a subclass of every class.


An enumeration is a daml:oneOf element, containing a list of the objects that are its instances.
This enables us to define a class by exhaustively enumerating its elements. The class defined by the oneOf element contains exactly the enumerated elements, no more, no less. For example:

<daml:oneOf parseType="daml:collection">
  <daml:Thing rdf:about="#Eurasia"/>
  <daml:Thing rdf:about="#Africa"/>
  <daml:Thing rdf:about="#North_America"/>
  <daml:Thing rdf:about="#South_America "/>
  <daml:Thing rdf:about="#Australia"/>
  <daml:Thing rdf:about="#Antarctica"/>

Property restrictions

A property restriction is a special kind of class expression. It implicitly defines an anonymous class, namely the class of all objects that satisfy the restriction. There are two kinds of restrictions. The first kind, ObjectRestriction, works on object properties, i.e., properties that relate objects to other objects. The second kind, DatatypeRestriction, works on datatype properties, i.e., properties that relate objects to datatype values. Both kinds of restrictions are created using the same syntax, with the usual difference being whether a class element or a datatype reference is used. It is also possible to create restrictions that are neither restrictions nor datatype restrictions, but these restrictions are not handled within DAML+OIL.

A daml:Restriction element contains an daml:onProperty element, which refers to a property name (a URI) (we will refer to this property as P) and one or more of the following

When there are multiple restrictions listed as part of a single Restriction element, the property P has to satisfy all of the restrictions (i.e., multiple restrictions are read as a conjunction).

Notice that the restrictedBy element which was associated with slot-restrictions in earlier versions of the language has now been removed, since it is completely synonymous with subClassOf.

Boolean combination of class expressions

A boolean combination of class expressions can be constructed from:

Note that arbitrarily complex combinations of these expressions can be formed.See Boolean Notefor an example of this.

Property elements

A rdf:Property element refers to a property name (a URI) (to which we will refer as P). Properties that are used in property restrictions should be either properties, which relate objects to other objects, and are instances of ObjectProperty; or datatype properties, which relate objects to datatype values, and are instances of DatatypeProperty.

A property element contains:

Instead of an object property or datatype property element, it is also possible to use any of the following elements, each of which assert additional information about the property:

Notice that UniqueProperty and UnambiguousProperty specify global cardinality restrictions. That is, no matter what class the property is applied to, the cardinality constraints must hold, unlike the various cardinality properties used in property restrictions, which are part of a class element, and are only enforced on the property when applied to that class.
A property is a binary relation that may or may not be defined in the ontology. If it is not defined, then it is assumed to be a binary relation with no globally applicable constraints, i.e. any pair with first element an object and second element an object or datatype value could be an instance of the property.
Warning: If a transitive property (or any of its superproperties) is used in a cardinality constraint, then class consistency is no longer necessarily decidable. Of course, UniqueProperty is a a particular case of a cardinality constraint.


Instances of both classes (i.e., objects) and of properties (i.e., pairs) are written in RDF and RDF Schema syntax.
See the specification of these languages for more details on the various syntactic forms that are allowed. Here we list just some of the most common notations:

<continent rdf:ID="Asia"/>

<rdf:Description rdf:ID="Asia">
   <rdfs:Class rdf:about="#continent"/>

<rdf:Description rdf:ID="India">
  <is_part_of rdf:resource="#Asia"/>

There is no unique name assumption for objects in DAML+OIL. To state that objects are the same, a daml:sameIndividualAs element is used. (Note that daml:equivalentTo can be also used here, but daml:sameIndividual is preferred. To state that objects are distinct, a daml:differentIndividualFrom element is used. The situation is different for datatype values, where XML Schema Datatype identity is used.

Datatype Values

Datatype values are written in a manner that is valid RDF syntax, but which is given a special semantics in DAML+OIL. The preferred method is to give a lexical representation of the value as a string, along with an XML Schema datatype that is used to provide the type of the value as well as the parsing mechanism to go from the string to the value itself. The XML Schema datatype is the rdf:type of the value, and the lexical representation is the rdf:value of the value. So the decimal 10.5 could be input as <xsd:decimal rdf:value="10.5"> provided that xsd was defined as the URI of the XML Schema Datatype specification.

As a nod to backward compatability, literals that occur outside this sort of construction are interpreted as any of the XML Schema Datatype values with this lexical representation. These values are mostly unusable unless some typing information is available, such as a range for a property.

The question of whether any XML Schema datatype can be used in such constructions, or whether only certain XML Schema dataypes can be so used (such as only the predefined datatypes), remains open.


DAML+OIL needs to represent unordered collections of items (also known as bags, or multisets) in a number of constructions, such as intersectionOf, unionOf, oneOf, disjointUnionOf and Disjoint. DAML+OIL exploits the rdf:parseType attribute to extend the syntax for RDF with a convenient notation for such collections. Whenever an element has the rdf:parseType attribute with value "daml:collection", the enclosed elements must be interpreted as elements in a list structure, constructed using the elements List, first, rest and nil.

For example, the statement

<oneOf rdf:parseType="daml:collection">
  <Thing rdf:resource="#red"/>
  <Thing rdf:resource="#white"/>
  <Thing rdf:resource="#blue"/>

should be interpreted as the following construction (also known as a consed-pair construction, from Lisp-lore):

  <Thing rdf:resource="#red">
    <Thing rdf:resource="#white">
      <Thing rdf:resource="#blue">
      <List rdf:resource="">

Current RDF parsers (RDF specification of February '99) will not support the daml:collection parseType. In order to process DAML+OIL documents, such parsers will have to be extended, or a separate preprocessing stage is required which translates the first form above into the second before the DAM+OIL code is given as input to the RDF parser.

Note that structures of parseType daml:collection are intended to represent unordered collections, even though the RDF datastructure imposes a specific order on the elements.

Appendix: Index of all language elements


Syntax Note:
As a simple example of an alternative syntactic form resulting in the same set of RDF triples, consider the statement in this document that "a DAML+OIL class definition consists at least of an rdfs:class element", which suggests the following syntactic form:
<rdfs:Class ID="Continents"/> 

However, the following RDF statement:

<rdf:Description ID="Continents">
  <rdf:Type resource=""/>

results in exactly the same set of RDF triples, and is therefore perfectly allowed as a class definition.

Another example is the two notations that we discuss for cardinality constraints below. Again, both these forms result in the same set of RDF triples, and are thus equivalent.

Mixing Note:
For example, take the class definition for Person from daml+oil-ex.daml, and then add
<rdf:Description about="#Person">
     <Creator>Ora Lassila</Creator>

then the semantics don't say what this means or what it would imply for instances of Person. (Beyond of course the minimal Subject-Verb-Object semantics of RDF).

Cardinality Syntax Note:
As an example of content-hiding syntax for cardinality expressions, instead of the standard notation:
  <onProperty rdf:resource="#father"/>

we would have to write

<Restriction cardinality="1">
  <onProperty rdf:resource="#father"/>

to avoid any exposed content. The cardinality elements are the only ones for which this alternative notation is required to avoid exposed content.(See the section on abbreviated syntaxin the RDF specification for more details on this notation)

Boolean Note:
As an example of a combination of boolean operators, the expression "neither meat nor fish" could be written as:
  <unionOf parseType="daml:collection"> 
   <Class rdf:resource="#meat"/>
   <Class rdf:resource="#fish"/> 

Disjoint Note:
For example, the following combination a Disjoint element with class expressions to state that "meat or fish" is disjoint from "plants or parts of plants".
<Disjoint parseType="daml:collection">
  <unionOf parseType="daml:collection">
   <Class rdf:resource="#meat"/>
   <Class rdf:resource="#fish"/>
  <unionOf parseType="daml:collection">
   <Class rdf:resource="#plant">
    <onProperty rdf:resource="#is_part_of">
    <hasValue rdf:resource="#plant">

$Revision: 1.10 $ of $Date: 2001/04/11 16:27:53 $.