 Given a set of equations \(\iEq(p, q)\) in two variables, this set defines on every algebra \(\bA\) a binary relation, namely the set of pairs \(\langle a, b\rangle\) of elements of \(A\) that satisfy in \(\bA\) all the equations in \(\iEq(p, q)\). In standard model-theoretic notation, this set is the relation