The logics \(\bL\) with the property that there is a set of formulas (possibly infinite) \(\Delta(p, q)\) in two variables that defines in every \(\bL\)-theory \(T\) its Leibniz congruence, that is, that for all \(L\)-formulas \(\phi , \psi\) it holds \[\langle \phi , \psi \rangle \in \bOmega_{\bFm }(T)\txtiff \Delta(\phi , \psi) \subseteq T,\] are known as the equivalential logics. If \(\Delta(p, q)\) is finite, the logic is said to be finitely equivalential. A set \(\Delta(p, q)\) that defines in every \(\bL\)-theory its Leibniz congruence is called a set of equivalence formulas for \(\bL\). It is clear that every algebraizable logic is equivalential and that every finitely algebraizable logic is finitely equivalential.