Thermodynamics - Introduction to Phase, p-v and T-v Diagrams

[@soumenpanda6404]

Nice explanation

[@marcgarrett2427]

"Pressure remains constant through the two phase region". 1) is this true of all phase diagrams or just T-V, and true of all two phase regions, or just liquid-vapor? 2) what is the physical reasoning behind pressure staying constant in the two phase region?