A reversible reaction that involves the conversion of two liquid phases, ${\text{l}}^{{}^{\prime }}$ and ${\text{l}}^{{}^{″}}$, into a solid phase α on cooling: $${\text{l}}^{{}^{\prime }}+{\text{l}}^{{}^{″}}\rightleftharpoons \text{α}$$ The maximum temperature at which this reaction can occur is the congruent melting point of the solid phase. Example: The conversion of co-existing K-rich and Zn-rich phases in the K-Zn system to form an intermediate solid phase KZn₁₃.