Summary: Basalt composition is defined as that chemical composition which when crystallized at one atmosphere under standard conditions will yield olivine, clinopyroxene, and plagioclase as the principal silicate phases. This gives six classes of basalt defined by their order of crystallization but only four of these are encountered in rocks: class 1, olivine→clinopyroxene→plagioclase; class 2, olivine→plagioclase→clinopyroxene; class 3, plagioclase→olivine→clinopyroxene; and class 4, plagioclase→linopyroxene→olivine. Classes 5 and 6 would have clinopyroxene as the first phase. The class can be determined from the rock composition from multivariate discriminant function equations but Si, Mg, Al, and Na provide a more simple diagnosis via dis-criminant diagrams. Rock series plotted on these diagrams show systematic trends that allow the series them-selves to be classified. Each class corresponds with a commonly recognized basalt type and all four classes relate to tectonic setting groups.