Goldschmidt’s (1911) study of the contact metamorphism of the Oslo area and Eskola’s (1914) study of the regional metamorphism of the Orijarvi region revealed that well-recrystallized metamorphic rocks approached chemical equilibrium at a stage of their history and that the mineral compositions they acquired then have usually been preserved over long subsequent periods of time. In a group of metamorphic rocks that have reached chemical equilibrium under the same definite P-T conditions, the mineral compositions of the rocks depend only on their bulk chemical compositions, regardless of their previous history. There exists a regular relationship between the mineral and chemical compositions of such metamorphic rocks. Eskola (1915) attempted to visualize the relationship by his ACF and A’KF diagrams.