In a ternary phase diagram, what occurs at the point where a compound exhibits congruent melting, and how does this relate to the triple point of the same substance?

In a ternary diagram a congruent melting point is where the liquidus and solidus lines intersect at the same composition, meaning the solid melts directly into a liquid of identical composition without any change in phase proportions. At this point the compound behaves as a pure substance, so the solid, liquid, and any third phase (often another solid or the vapor) can coexist at a single temperature and pressure, forming a triple point. Thus, the congruent melting point is essentially the same as the triple point for that substance because the solid, liquid, and the third phase meet simultaneously. For example, pure NaCl in a NaCl–Na2CO3–NaOH diagram melts congruently at its melting temperature, and the NaCl solid, NaCl liquid, and NaCl vapor all coexist at that same temperature, marking both the congruent melting point and the triple point.