This paper presents an extension of the Multiple Grammars Theory (Roeper, 1999) to provide a formal mechanism that can serve as a generative-based alternative to current descriptive models of interlanguage. The theory extends historical work by Kroch and Taylor (1997), and has been taken into a computational direction by Yang (2003). The proposal is based on the idea that any human grammar readily accommodates sets of rules in sub-grammars that can seem (apparently) contradictory. We discuss the rationale behind this proposal and establish a dialogue with recent research in SLA, multilingualism, L3 acquisition, and L2 processing. We compare the Multiple Grammars explanation to optionality in L2 to other current proposals, and provide experimental results that can demonstrate the existence of active sub-grammars in the linguistic representation of L2 speakers.