The Si – Al ordering of plagioclase feldspars is discussed in terms of relative bond energies and the forms of the ordered plagioclases are predicted. Unmixing into two compositions is interpreted as an ordering phenomenon. Plagioclase crystallization is discussed as a kinetic-bond energy problem involving short-range order. It is predicted that certain compositions are more easily ordered, more easily crystallized, and form in different environments than closely related compositions. Certain compositions are favored and others are essentially prevented from forming. Frequency of compositional occurrence is tentatively correlated with these favored compositions.