  The present paper proposes a new introductory treatment of the very well
known Sahlqvist correspondence theory for classical modal logic. The first
motivation for the present treatment is {\em pedagogical}: classical Sahlqvist
correspondence is presented in a uniform and modular way, and, unlike the
existing textbook accounts, extends itself to a class of formulas laying
outside the Sahlqvist class proper. The second motivation is {\em
methodological}: the present treatment aims at highlighting the {\em algebraic}
and {\em order-theoretic} nature of the correspondence mechanism. The
exposition remains elementary and does not presuppose any previous knowledge or
familiarity with the algebraic approach to logic. However, it provides the
underlying motivation and basic intuitions for the recent developments in the
Sahlqvist theory of nonclassical logics, which compose the so-called unified
correspondence theory.
