# Class Axioms

Class Axioms are used to define classes, e.g. an <owl:class/> defines the existence of a class and is a class axiom, likewise is the ID of the class a class axiom.

The set of individuals linked to the class is called class extension.

## Contents

## SubClassOf axiom

It means the class expression is an instance of another class's expression, it is used to display hierarchy. It follows that the set of individuals in the class 1 are a subset of the set of individuals of class 2. Due to that the first class expression which is the subclass of the other is more specific than the other.

## Equivalent classes axiom

Equivalent class axioms state that multiple class expressions are equivalent to each other. Thus these class expressions can be used as synonym, when the meaning of the ontology won't be changed.

## Disjoint classes axiom

These axioms state that class expressions are disjoint, thus they have no instances in common.

## GCI

Counts the number of the General Concept Inclusion (GCI).

## HiddenGCI

Counts the number of "hidden" GCIs in an ontology imports closure. A GCI is regarded to be a "hidden" GCI if it is essentially introduce via an equivalent class axiom and a subclass axioms where the LHS of the subclass axiom is nameed. For example, A equivalentTo p some C, A subClassOf B results in a "hidden" GCI.