An ensemble of figures required for the analysis of the atomic–vacancy ordering in nonstoichio metric compounds M2Xy (M2Xy 1 – y) with an L'3 hexagonal structure has been determined by the order parameter functional method. It has been demonstrated reasoning from the crystal lattice geometry that, in order to describe M2Xy compounds with an L'3 structure, it is necessary to use the basis cluster in the form of a trigonal prism consisting of six sites of the nonmetal lattice with a metal atom at the center. The overlapping figures that, together with the basis cluster, form a sequence of special figures uniquely describing the L'3 structure have been found. The equilibrium conditions for disorder–order transformations in nonstoichio metric compounds M2Xy have been determined in the general form.