What is the abbreviation for underlying representation? 

Underlying representation

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 applied to it. By contrast, a surface representation is the phonetic representation of the word or sound. The concept of an underlying representation is central to generative grammar.If more phonological rules apply to the same underlying form, they can apply wholly independently of each other or in a feeding or counterbleeding order. 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.

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