Abstract
Assimilation is a central phenomenon in phonology, yet there is little consensus on either its representation or computation. In particular, the empirical distinction between spreading (feature sharing) and correspondence (feature copying) is disputed. In this paper, I identify novel diagnostics from two interacting assimilation processes in San Francisco del Mar Huave (isolate: Mexico). First, vowel-copy epenthesis displays a previously unattested blocking pattern that is problematic for spreading, but predicted by feature-copying approaches like Agreement By Correspondence. Second, in CV agreement, I argue that only feature sharing driven by Dep and Specify constraints can insightfully account for the role of underspecification, which produces a range of directionality effects. Huave shows that both spreading and correspondence are needed in phonological theory, and also demonstrates that monolithically assimilation-mandating constraints like Agree can be decomposed to derive assimilation from the interaction of more elementary, independently motivated principles of markedness and faithfulness.