What pressure is represented by this pV diagram? A common pV shown in physics textbooks is the one below for a gas that deviates from ideal behavior:

(Taken from University Physics by Young and Freedman.)
For the $T_4$ and $T_3$ curves, the pressure on y-axis is the gas pressure. However, for the other curves, what pressure is being referred to? You have two phases, and the pressure of a liquid is not uniform throughout its bulk. Is the P value indicative of the pressure at the liquid’s surface? At the liquid’s lowest point? Is it its average pressure?
 A: You have a closed contener with a piston within vapor of a certain substance at constant temperature. Now you start to push the piston so that the volume decreases.
Now take for example the second curve(T2) and go towards the minimum of volume(direction opposite to the x-axis)pushing the piston. First, in the right part there is only vapor and the pressure is exactly the pressure exercised by the vapor at that volume(equal to the pressure exercised by the piston). When the vapor arrive at point "a" it starts to condense. Now you continue to push the piston to reduced the volume, but while there is at the same time vapor and liquid the pressure that you need to apply is exactly the vapor pressure of the gas in equilibrium with the liquid and this vapor pressure is constant during all the phase transition because the temperature is constant. 
Proceedings you will arrive at point "b" where all the vapor has become liquid and the phase transition is concluded. Now your piston is directly in contact with the liquid phase because there is no more gas above the liquid. And if you want to reduce the volume of the liquid you have to exercised an enormous pressure(left part of the graphic). 
A: The figure inherently neglects the effect of gravity, so that the pressure is taken to be uniform throughout.  This is often the case is vessels that are short vertically.  The volumes shown in the figure are usually specific volumes, equal to the local volume per unit mass (so that, when liquid and vapor are both present, each phase has its own specific volume at the ends of the horizontal lines segments).  The average specific volume of the mixture is a weighted average of the liquid and vapor values.
