The SD provides information (to be interpreted by performance systems) about the properties of each linguistic expression, including its sound and its meaning. We assume that the design of language provides a variety of symbolic systems ("levels of representation") fulfilling these tasks, including the level of "Phonetic Form" (PF) and the level of "Logical Form" (LF), specifying aspects of sound and meaning, respectively, insofar as they are linguistically determined. Another is the level of D-Structure, which relates the computational system and the lexicon.
- Chomsky (1995), a pag.21