A diagram that links the stability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemblage stability ranges are commonly plotted as reaction boundaries in pressure–temperature space to produce a petrogenetic grid for a particular range of rock compositions. The region of overlap of the stability fields of those minerals forming the equilibrium mineral assemblage define the pressure–temperature conditions of the equilibrium. This corresponds to the pressure and temperature of the metamorphism.