Lexically linked domains in language allow a speaker to formulate incompatible rules. How should they be represented theoretically? We argue that a speaker has a set of mini-grammars for different domains so that, in effect, every speaker is bilingual. It is argued that Tense or Agreement Checking, V-2 for quotation, and resumptive pronouns, all lead to bilingual representations. In addition, this perspective on Theoretical Bilingualism suggests that optionality and stages in the acquisition of an initial grammar should also be characterized as a form of bilingualism.