Chapter 5 (‘Stems’) is a detailed account of the role of stems in the definition of inflectional paradigms. In the simplest cases, the same stem serves as the basis for every word form in a lexeme’s paradigm. But a lexeme’s inflection often depends on more than one stem. Stem alternations within a lexeme’s inflectional paradigm are of various kinds. Sandhi alternations are purely an effect of automatic phonology. Among nonautomatic alternations, some are phonologically conditioned and others grammatically conditioned; cross-cutting this distinction is a distinction between between class-determined alternations (alternations that follow from membership in a particular inflection class) and class-independent alternations (alternations that are not simply an effect of membership in a particular inflection class); and grammatically conditioned stem alternations may themselves morphosyntactically conditioned (in which each alternant is invariably associated with a particular morphosyntactic property set) or morphomic (Aronoff 1994), following a distributional pattern whose significance is purely morphological, with no invariant phonological, syntactic or semantic correlate.