In this paper, I address the nature of the mental representation of an interlanguage grammar. The focus will be on the necessity of positing some sort of hierarchical constituent structure to account for L2 phonology. I discuss relevant data from the domains of the acquisition of segments, syllables, moras, and metrical structure. The interaction of these domains is discussed.

In addition, I look at the acquisition of onset clusters and argue that the acquisition of liquids is correlated with the acquisition of consonantal sequences. Evidence from language change, language typology, and language acquisition suggests that there is a causal relationship between the two. The theoretical framework of feature geometry and derived sonority gives us the apparatus to explain what the second-language learners are doing.