This book presents rigorous and criterial definitions of the major parts of speech - noun, verb, and adjective - that account both for their syntactic behaviour and for their observed typological variation. Based on an examination of languages from five different groups - Salishan, Cora, Quechua, Totonac, and Hausa - this book argues that parts of speech must be defined by combining the criteria of syntactic markedness, which characterizes lexical classes in terms of unmarked syntactic roles, and semantic prototypicality, which delimits their prototypical meanings. Adjectives are shown to be the marked (and, hence, most variable) class because of their inherent non-iconicity at the semantics/syntax interface. The four-member typology of parts of speech systems (languages with three open classes, those that group adjectives with verbs, those that group adjectives with nouns, and those that conflate all three) current in the literature is easily generated by free recombination of these two criterial features. Closer examination of the data, however, casts doubt on the existence of one of the four possible language-types, the noun-adjective conflating inventory, which is accounted here for by replacing free recombination of semantic and syntactic features with an algorithm for the subdivision of the lexicon that gives primacy to semantics over syntax.