Another obscure rule is explained here. The idea of degrees-of-freedom is also used in the State Postulate but the SP and the GPR are independent of each other. This information is based on the work of Professor WR Salzman who has licensed it for non-commercial reuse (such as this post). I have tried to contact Salzman, to thank him for his beautiful explanations, but thus far to no avail.
(In the diagram above there is an image with some ‘billiard balls’ (red and black blobs) connected by springs. I realise that this needs some explanation. It is intended to represent the molecular behaviour of a cube of solid ice in liquid water. At 0 degC, the cube is in phase equilibrium with the liquid. Some molecules within the solid structure become liquid water molecules and an equal number of liquid molecules join the solid structure. At -5 degC, on the left, the liquid molecules have higher chemical potential energy in their bonds, than those in the solid phase and so there is a net tendency for them to move to a lower-potential state and become solid (freezing). The opposite is the case at +5 degC (where melting occurs). At equilibrium, the chemical potential of each phase is the same).