Linguistic theory provides a (universal) system of 'levels of representation'. Each level 'L' is a system based on a set of primes (minimal elements - i.e., an alphabet); the operation of concatenation, which forms strings of primes of arbitrary finite lenght [...]; various relations; a designated class of strings (or set of strings) of primes called L-markers; a mapping of L-markers onto L'-markers, where L' is the next "lower" level (thus levels are arranged in a hierarchy). In particular, on the level P of phrase structure and the level T of transformations we have P-markers and T-markers in the sense just described informally. A hierarchy of linguistic levels (phonetic, phonological, word, morphological, phrase structure, transformational structure) can be developed within a uniform framework in this way.
- Chomsky (1969), Pag. 222-223