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.[1][2] 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.

2. 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.[1][2] 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.

3. UNDERLYING REPRESENTATION
• 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.

4. EXAMPLES
• IN MANY CASES, THE UNDERLYING FORM IS SIMPLY
THE PHONEMIC FORM. 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/.)

5. EXAMPLES
• PHONOLOGICAL RULES MAY CHANGE THE PHONEMES INVOLVED. IN SUCH
CASES, PIPES ("|") OR DOUBLE SLASHES MAY BE USED IN TRANSCRIPTION TO
DISTINGUISH THE UNDERLYING FORM FROM ITS PHONEMIC REALIZATION. FOR
EXAMPLE, THE WORD "CATS" HAS THE PHONEMIC REPRESENTATION /KÆTS/. IF
IT IS ASSUMED THAT THE UNDERLYING FORM OF THE ENGLISH PLURAL SUFFIX
IS A [Z] SOUND, THE UNDERLYING FORM OF "CATS" WOULD BE //KÆTZ//.
(THE [Z] SURFACES AS AN [S] BECAUSE OF THE PHONOLOGICAL PROCESS OF
DEVOICING AFTER AN UNVOICED CONSONANT.)

6. HTTPS://EN.WIKIPEDIA.ORG/WIKI/UNDERLYING_REPRESENTATION
THANK YOU