Abstract
A system with three or more components (n $$ \geqslant $$ 3) is quasi-simple if the isotherms–isobars–isopotentials of the system’s components (the solvents of solubility diagrams) are line segments (n = 3), sectors of planes (n = 4), or hyperplanes (n $$ \geqslant $$ 5). This work presents evidence that all figurative points on monovariant branches of the joint crystallization of two identical solid phases in different quasi-simple quaternary systems (n = 4) (with two identical and one different component) belong to a single curve. Systems of equations are given for thermodynamic calculations of such curves exclusively from data on binary subsystems. Examples of calculating these monovariant curves are provided, and agreement is shown between non-model calculations and experimental data on the compositions of saturated solutions using quaternary systems with a common cation, a common anion, and quaternary reciprocal systems as examples. Agreement is also seen between results from thermodynamic calculations and numerous experimental data. The results can naturally be extended to monovariant crystallization lines of three or more identical solid phases on the solubility diagrams of quasi-simple systems with five or more components (and one that is different).