Let R = {(x, y) : y = 3x - 1} be a relation from A = {1, 2, 3, 4} to ℕ.

$A \times B$ holds every pair $(a, b)$, and $n(A \times B) = n(A)\,n(B)$. A relation is any subset of $A \times B$. On one set, a relation that is reflexive, symmetric and transitive is an equivalence relation.