Combination of the oxide components of an igneous rock into a set of water-free, standard, mineral compounds (termed ‘normative constituents’) according to a rigidly prescribed order that originally was thought to be the order of mineral crystallization in most magmas. The calculation, developed in the late 19th century by the American petrologists W. Cross, J. P. Iddings, L. V. Pirsson, and H. W. Washington, provides a basis for comparing and classifying rock types independently of their modal mineral assemblages. Rocks of similar chemistry may develop contrasting modal mineral assemblages if they crystallize under contrasting pressure, P_total, and P(H₂O) conditions. Use of the CIPW norm calculation eliminates these effects, allowing a comparison based on an ideal mineral assemblage controlled only by the original magma chemistry.