In early transformational syntax, deep structures are
derivation trees of a
context-free language. These trees are then transformed by a sequence of tree rewriting operations ("transformations") into surface structures. The
terminal yield of a surface structure tree, the surface form, is then predicted to be a grammatical sentence of the language being studied. The role and significance of deep structure changed a great deal as Chomsky developed his theories, and since the mid-1990s deep structure no longer features at all (see
minimalist program). It is tempting to regard deep structures as representing meanings and surface structures as representing sentences that express those meanings, but this is not the concept of deep structure which Chomsky favoured. Rather, a sentence more closely corresponds to a deep structure paired with the surface structure derived from it, with an additional
phonetic form obtained from processing of the surface structure. It has been variously suggested that the interpretation of a sentence is determined by its deep structure alone, by a combination of its deep and surface structures, or by some other level of representation altogether (
logical form), as argued in 1977 by Chomsky's student
Robert May. Chomsky may have tentatively entertained the first of these ideas in the early 1960s, but quickly moved away from it to the second, and finally to the third. Throughout the 1960s and 1970s, the
generative semantics movement put up a vigorous defence of the first option, sparking an acrimonious debate, the "
Linguistics Wars". Chomsky noted in his early years that by dividing deep structures from surface structures, one could understand "slip of the tongue" moments (where someone says something that they did not intend) as instances where deep structures do not translate into the intended surface structure. ==Extension to other fields==