How does specific volume play its role in the phase diagram?

[Carbon273]

So, I am casually sifting through a chapter in a thermodynamics textbook talking about the multiphase process that pure substances go through. I understand how the P-v and T-v diagram works and that out of the three properties (specific volume, temperature and pressure), two of them are independent, describing a state postulate. However, I am having a bit of trouble understanding how specific volume plays a role in the P-T diagram. I understand that there is 3-dimensional relation between the three properties called the P-v-T relation and that the P-v diagram, the T-v diagram, and the P-T diagrams are mere 2D projections on the 3D relation. For the P-T diagram, it seems like the specific volume is held constant at any P-T combination seeing that it goes into or out of the page. Can somebody clarify this for me, if I am in the right track?

 

[Chestermiller]

That is certainly a correct interpretation.