In some models of phonology as well as morphophonology in the field of linguistics, the underlying representation (UR) or underlying form (UF) of a word or morpheme is the abstract form that a word or morpheme is postulated to have before any phonological rules have been 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 /t/ in a word like wet can surface either as an unreleased stop [t̚] or as a flap [ɾ], depending on environment: [wɛt] wet vs. [ˈwɛɾɚ] wetter.
(In both cases, however, the underlying representation of the morpheme wet is the same: its phonemic form /wɛt/.)
In such cases, pipes ("|") or double slashes may be used in transcription to distinguish the underlying form from its phonemic realization.