Underlying representation

In phonology, the underlying representation or underlying form of a morpheme is the abstract form the morpheme is postulated to have before any phonological rules have applied to it. The underlying representation of a morpheme is considered to be invariable across related forms (except in cases of suppletion), despite alternations among various allophones on the surface.

For example, in many varieties of American English the phoneme in a word like wet can surface either as a glottalized  or as a flap, depending on environment:  wet vs. wetter. In both cases, however, the underlying representation of the morpheme wet is the same:.