The performance systems appear to fall into two general types: articulatory-perceptual and conceptual-intentional. If so, a linguistic expression contains instructions for each of these systems. Two of the linguistic levels, then, are the "interface levels" A-P and C-I, providing the instructions for the articulatory-perceptual and conceptual-intentional systems, respectively. Each language determines a set of pairs drawn from the A-P and C-I levels. The level A-P has generally been taken to be PF.
- Chomsky (1995), a pag.168

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