Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!
The standard way of defining a relation R between a set X and a set Y is as some subset of (X x Y). We write aRb if and only if (a,b) is in R. Some particularly important kind of relations are MappinG''s'', EquivalenceRelation''s'', and PartialOrder''s''.