A notion that is central to transformational grammar’s (Chomsky 1965) description of the syntax of natural language. Deep structure is where the predicate—argument relationships are expressed, and both relates the words of a sentence to its meaning and expresses grammatical generalizations. Within the ‘standard theory’ of transformational grammar the context-free base generates deep structures and a set of transformations that map these to the surface structure. In natural-language processing, sentences can be analysed by either fully or partially running transformations in reverse.