This article offers commentary on the Multiple Grammars (MG) language acquisition theory proposed by Luiz Amaral and Tom Roeper in this issue. It argues that more precise definitions are needed for the terms "rule," "simple," and "productive." Topics discussed include Amaral and Roeper's verb second (V2) rule, the concept of optionality, and the effect prior language knowledge has on second language grammars. It addresses such problems as how to determine the correct level of analysis for deciding what a "rule of grammar" is, how "simple" it is, and whether rules and even features are exactly comparable cross-linguistically.