In this paper, we have considered some elements of the classical phenomenological theory
of thermodynamic stability, which seem controversial and ambiguous. The main focus is on the
conditions of the stability boundary; a new version of the derivation of the relations defining the
specified boundary is proposed. Although the final results, in general, coincide with the classical
relations, the described approach, from our point of view, provides a clearer and more accurate idea
of the stability conditions and their boundaries.