The standard way of defining a relation R between a SeT X and a SeT Y is as some SubSet of the CartesianProduct? (X x Y). We write aRb if and only if (a,b) is in R. This can also be looked at as a mapping from (X x Y) to 2={0,1}.

Some particularly important kind of relations are MappinG*s*, EquivalenceRelation*s*, and PartialOrder*s*.