Consider now the coexistence curve between the solid and liquid phases.
This is depicted as the fusion curve AB in Fig.3.4.
Note that the fusion curve does not terminate.
It can have either positive or negative slope, as depicted with a solid and dotted line in Fig.3.4, respectively.
The relevant Clausius-Clapeyron equation is
where is the molar latent heat of fusion. Hence, the sign of determines that of the slope of the fusion curve. A well known example of a fusion curve that has a negative slope is that between ice and water.