Classifiers and noun class markers are often semantically general and semantically opaque compared to open-class nouns, and in this sense they constitute a semantic reduction of the noun universe. These two semantic characteristics also play important roles in the diachronic development of nominal classification systems. First, the need for semantically general forms for anaphoric reference may be a possible motivation for developing nominal classification in the first place. Second, opaque classification, which may, for example, emerge through coales-cence of classes with homophonous markers, may be replaced by transparent classification because of the incompatibility of opaque classification and certain syntactic constructions, such as contrastive focus. Finally, opaque classification, typical of grammatical gender systems, is less likely to diffuse through language contact than transparent classification, which is typical for other types of systems , including numeral classifier systems.