 We start by describing the first two steps involved in the algebraic study of propositional logics. Both are needed in order to endow propositional languages with algebraic interpretations. To expound them we will assume knowledge of first-order logic (see the entries on  classical logic  and  first-order model theory)  and we will call algebraic first-order languages, or simply algebraic languages, the first-order languages with equality and without any relational symbols, so that these languages have only operation symbols (also called function symbols), if any, in the set of their non-logical symbols.