A rule for a joint of three boundary lines in phase diagrams

The rule governing the mutual position of three boundary lines meeting at one point in a two-dimensional phase diagram is derived in a new way using the fact that a boundary can be extended into the metastable region beyond the point of its intersection with another line. An improved formulation of the rule is proposed, which makes it independent of the diagram axes chosen and suitable for the analysis of two-dimensional sections of phase diagrams of heterogeneous systems with any number of components. Conditions for and some aspects of the application of the rule are considered using phase diagrams of one-, two-, and three-component systems as examples.