When analyzing a phase diagram, what do the lines representing phase boundaries indicate about the system's conditions?

The lines on a phase diagram show the exact temperature and pressure at which two different phases can coexist in equilibrium. They mark the boundary where the system can be in either of the two phases or a mixture of both at the same time. When a point lies on one of these lines, the material is at the precise conditions needed for the two phases to coexist. For example, the liquid–vapor line for water indicates the boiling point at each pressure, so at 1 atm the line is at 100 °C. Thus, the phase‑boundary lines tell us the conditions that keep the system stable in those two phases.