Suppose both our agents have access to an ontology for travel, with concepts such as airplanes and destinations, and suppose their agent tells our agent about a flight on a 777. The travel agent's agent could explain that a 777 is a kind of airplane, which is a concept in the travel ontology. Our agent would then know the general characteristics of a 777. This communication is illustrated in Figure 2 . Communication between agents sharing a travel ontology.
Most ontologies represent and support relationships among classes of meaning.
Generalization is the relationship between a class and one or more refined versions of it. Each subclass inherits the features of its superclass, adding other features of its own. Generalization and inheritance are transitive across an arbitrary number of levels. They are also antisymmetric. Aggregation is also transitive, as well as antisymmetric.