underlying form
Definition - The abstract form a morpheme is postulated to have before any phonological rules have been applied to it. In other words, it is the phonemic form that your mind wants to say; as opposed to the phonetic or surface form that results when you actually say it.

Example -
In American English the underlying phoneme /t/ in a word like wet (/wɛt/) can surface either as a glottalized [tˀ] or as a flap [ɾ], depending on environment, thus resulting in the spoken surface forms [ˈwɛtˀ] wet or [ˈwɛɾɚ] wetter.

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