The Cluster Variation method with Natural Iteration Algorithm is used to compute isothermal sections of several typical phase diagrams for clustering and ordering ternary fcc solid solutions, given as only input, the three nearest-neighbor pair interaction parameters. A comparison is made with phase diagrams corresponding to Meijering's eight regular solution categories. In categories IA-IIIB, ordering must take place at sufficiently low temperature so that ordered phase regions must interact with miscibility gap boundaries, as shown by means of several examples calculated hers in the nearest-neighbor tetrahedron cluster variation approximation. The chemical potential diagrams are also shown, and the orthogonality theorem is proved which says that a tie line is perpendicular to the chemical potential curve at the corresponding point. © 1977.