To take a general case, let A and B be the two components and AB a stable solid compound formed by their chemical combination.
Accordingly, there will be three fusion or freezing point curves AC, BE and CDE for the three solid phases.
Evidently, at this temperature, the two-component system has become a one-component system because both solid and liquid phases contains only the compound AB.
[1][2] Congruent melting point represents a definite temperature just like the melting points of pure components.
In some phase diagrams, the congruent melting point of a compound AB may lie above the melting points of pure components A and B.