X and Y are variables used for writing axioms about roles (such as be-child-of) in a form of If ... then ... rules or ... if-and-only-if ... rules. Those axioms are OWL expressions. The axioms include: general role inclusion, transitivity etc. The axioms modify semantic sense of the roles. All of them are in fact a kind of semantic sugar and can be thought of as special cases of a general role inclusion and concept subsumption axioms.


The examples below show how to write simple axioms about roles using variables X and Y.

Example: General role inclusion.
If X is-daughter-of Y then X is-child-of.
Example: Role transitivity.
If X has-part something that has-part Y then X has-part Y.
Example: Symmetric role.
X has-sibling Y if-and-only-if Y has-sibling X.