On a phase diagram, the lines are "lines of equilibrium", i.e. at any point on these lines of equilibrium there is an established equilibrium between the two phases on either side of the line. The equilibrium constant is a function of temperature and pressure, and the various other thermodynamic factors (e.g. specific volume, specific enthalpy, or specific entropy) you may plot phase diagrams for. However the equilibrium does not just spring up at this boundary which the line represents. Either side of it, the equilibrium must still be occurring, but less appreciably. For instance, liquids usually have a liquid-vapour equilibrium which I presume would fall under this category too. Thus, my first question is, what values or range of the equilibrium constant do the lines in standard phase diagrams tend to represent? e.g. looking at the phase diagram for water, http://en.wikipedia.org/wiki/File:Phase-diag2.svg, the blue line in the diagram represents the equilibrium line for the equilibrium H2O (l) <-> H2O (g), i.e. at the points of (T,P) along the line is this equilibrium constant within some appreciable range which means the phases are considered at equilibrium. What is this range of values?