Sometimes a morpheme has more than one shape, depending on the environment in which it occurs. The shape of a morpheme may be affected by nearby sounds, by the kind of stem it is attached to, or by other conditioning factors. The systematically distinct shapes of a morpheme are called its allomorphs. When a morpheme changes its shape in response to the sounds that surround it in a particular context, linguists often call the variation morphophonemics (or morphophonology), and the patterns that describe the appearance of the allomorphs morphophonemic rules (or morphophonological rules). Morphophonemic rules are very different from the morphological rules described in chapters 1 and 2, and should always be kept quite distinct conceptually. Morphophonemic rules do not express conceptual categories. Rather, they simply specify the pronunciations (the “shapes”) of morphemes in context, once a morphological rule has already applied.

Morphophonemics can also be thought of as the interface between phonology and morphology. Phonological patterns (or “rules”) specify the pronunciation of sounds in particular environments. They only make reference to sequences of sounds, and not to whether those sequences involve particular morphemes or not. Morphophonemic patterns in most cases are just phonological patterns that come into play when morphemes come together in words. Occasionally, however, there are morphophonemic patterns that only apply when certain morphemes come together. These are morphophonemic patterns that are not, strictly speaking, phonological patterns.