Abstract In this paper I argue that cross-linguistic similarity in third language acquisition is determined by a structural hierarchy of contrastive phonological features. Such an approach allows us formalize a predictive notion of I-proximity which also provides an explanatory model of L2, and L3 phonological knowledge (represented in an integrated I-grammar). The metrics of phonological similarity (i.e., structural not acoustic) are analogous to morphosyntactic similarity in that both morphosyntactic and phonological approaches can compare the outcomes of parsing the L3 input by the L1 hierarchy and by the L2 hierarchy. From this starting point I propose a conservative, incremental learning theory to guide subsequent reconstruction of the L3 grammar. Under this model, it can be argued that phonology is part of Faculty of Language Narrow (FLN). The (gradient) phonetic material comes from outside the FLN but the linguistic computational system converts it to discrete abstract elements that can be manipulated by the learner.