<P> The value of an attribute can be a complex data type; in this example, the related engine can only be one of a list of subtypes of engines, not just a single thing . </P> <P> Ontologies are only true ontologies if concepts are related to other concepts (the concepts do have attributes). If that is not the case, then you would have either a taxonomy (if hyponym relationships exist between concepts) or a controlled vocabulary . These are useful, but are not considered true ontologies . </P> <P> Relationships (also known as relations) between objects in an ontology specify how objects are related to other objects . Typically a relation is of a particular type (or class) that specifies in what sense the object is related to the other object in the ontology . For example, in the ontology that contains the concept Ford Explorer and the concept Ford Bronco might be related by a relation of type <is defined as a successor of>. The full expression of that fact then becomes: </P> <Ul> <Li> Ford Explorer is defined as a successor of: Ford Bronco </Li> </Ul>

What types of nodes are typically used in ontologies