Explain the Two solids and a solvent - Phase Diagram?

The phase diagram in Figure on the next page is for a ternary system of water and two salts with an ion in common. There is a one-phase area for solution, labeled sln; a pair of two-phase areas in which the phases are a single solid salt and the saturated solution; and a triangular three-phase area. The upper vertex of the three-phase area, the eutonic point, represents the composition of solution saturated with respect to both salts. Some representative tie lines are drawn in the two-phase areas.

Figure: Ternary phase diagram for NaCl, KCl, and water at 25 C and 1 bar. The dashed lines are tie lines in the two-phase areas.

A system of three components and three phases has two degrees of freedom; at fixed values of T and p, each phase must have a fixed composition. The fixed compositions of the phases that are present when the system point falls in the three-phase area are the compositions at the three vertices of the inner triangle: solid NaCl, solid KCl, and solution of the eutonic composition xNaCl = 0.20 and xKCl = 0.11. From the position of the curved boundary that separates the one-phase solution area from the two-phase area for solution and solid KCl, we can see that adding NaCl to the saturated solution of KCl decreases the mole fraction of KCl in the saturated solution. Al- though it is not obvious in the phase diagram, adding KCl to a saturated solution of NaCl decreases the mole fraction of NaCl.