[…] there are two "interface levels", Phonetic Form (PF) at the A-P interface and Logical Form (LF) at the C-I interface.
- Chomsky (1995), a pag.2

The level PF must satisfy three basic conditions of adequacy. It must be "universal", in the sense that an expression of any actual or potential human language is representable within it. It must be an "interface", in that its elements have an interpretation in terms of the sensorimotor system. And it must be "uniform", in that this interpretation is uniform for all laguages, so as to capture all and only the properties of the system of language as such. The same three conditions hold for LF.
- Chomsky (1995), a pag.21