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Mineralogical nomenclature in solid-solution series follows a system that has been called the 50% rule, more correctly the 100%/n rule or the dominant-constituent rule, in which the constituents are atoms (cations or anions), molecular groups, or vacancies. Recently developed systems of nomenclature for the arrojadite and epidote groups have shown that a group of atoms with the same valency state must also be considered as a single constituent to avoid the creation of impossible end-member formulae. The extension with this dominant-valency rule is imposed by all cases of coupled heterovalent–homovalent substitutions. End members with a valency-imposed double site-occupancy may result from single-site heterovalent substitutions and from coupled heterovalent substitutions at two sites where there is a disparity in the number of these two sites.