A reversible reaction that involves the conversion of two liquid phases, \(\rm{l}^{'}\) and \(\rm{l}^{''}\), into a solid phase $\ce{\upalpha\!}$ on cooling: \[\rm{l}^{'}+\rm{l}^{''}\rightleftharpoons \unicode[Times]{x3B1} \] The maximum temperature at which this reaction can occur is the congruent melting point of the solid phase. Example: The conversion of co-existing $\ce{K}$-rich and $\ce{Zn}$-rich phases in the $\ce{K-Zn}$ system to form an intermediate solid phase $\ce{KZn13}$.