[…] one might briefly characterize the syntactic theories that have arisen in modern structural (taxonomic) linguistics as based on the assumption that deep and surface structures are actually the same. The central idea of transformational grammar is that they are, in general, distinct and that the surface structure is determined by repeated application of certain formal operations called "grammatical transformations" to objects of a more elementary sort.
- Chomsky (1969), Pag. 16-17

A grammatical transformation is, in other words, a rule that applies to Phrase-markers rather than to strings in the terminal and nonterminal vocabulary of the grammar.
- Chomsky (1969), Pag. 89

[…] we can apparently define a grammatical transformation in terms of a "structure index" that is a Boolean condition on Analyzability and a sequence of elementary transformations drawn from a base set including substitutions, deletions, and adjunctions.
- Chomsky (1969), Pag. 147