[…] the syntactic component of a grammar must specify, for each sentence, a "deep structure" that determines its semantic interpretation and a "surface structure" that determines its phonetic interpretation. The first of these is interpreted by the semantic component; the second, by the phonological component.
- Chomsky (1969), a pag.16

Thus when we define "deep structures" as "structures generated by the base component", we are, in effect, assuming that the semantic interpretation of a sentence depends only on its lexical items and the grammatical functions and relations represented in the underlying structures in which they appear.
- Chomsky (1969), a pag.136

A deep structure is a generalized Phrase-marker underlying some well-formed surface structure.
- Chomsky (1969), a pag.138