I propose that there is a purely phonological drive to impose a reduplication-like structure (‘coupling’) on words. This structure can lead to enhancement or preservation of word-internal self-similarity. The case of vowel raising in Tagalog loan-stems is examined in detail. Raising can be blocked in order to preserve similarity between the stem penult and the stem ultima. The more similar the penult and ultima along various dimensions, the more likely coupling is, and thus the more likely resistance to raising. I attribute phonologically driven coupling to the activity of a constraint REDUP in generation, which shapes the way new words are lexicalised in the vowel-raising case, but also consider an alternative source for reduplicative construals (the effect of *SPEC in lexical learning). The proposal is compared to others that promote correspondence between similar or identical single segments within a word; I conclude that a relation between strings is necessary.