Explain Composition of individual phase in phase diagram – two phase?

If the system point is in a two-phase area of the phase diagram, we draw a horizontal tie line of constant temperature (on a temperature-composition phase diagram) or constant pressure (on a pressure - composition phase diagram). The lever rule applies. The position of the point at each end of the tie line, at the boundary of the two-phase area, gives the value of the composition variable of one of the phases and also the physical state of this phase: either the state of an adjacent one-phase area, or the state of a phase of fixed composition when the boundary is a vertical line. Thus, a boundary that separates a two-phase area for phases α and β from a one-phase area for phase α is a curve that describes the composition of phase α as a function of T or p when it is in equilibrium with phase β. The curve is called a solidus, liquidus, or vaporus depending on whether phase α is a solid, liquid, or gas.