projection of a 
-surface, at which three different phases (for example solid, liquid, and vapour) can coexist at the same pressure and temperature, each phase having the same molar volume (or density). [P7.2P7.4]
is: a point on the projection of a -surface, at which three different phases (for example solid, liquid, and vapour) can coexist at the same pressure and temperature, each phase having the same molar volume (or density). [P7.2P7.4]
Copyright 1997, The Open University