One of the key questions in the study of language concerns the architecture of the grammar. At issue is the nature of and relationship between the systems that generate linguistic output. The present paper tests the predictions of two competing theories of grammatical architecture for a variable process of h-deletion in connected speech. A usage-based theory of grammar predicts that this phonetic lenition rule will show word-sensitivity, while a modular, feed-forward theory predicts a uniform rate of h-deletion across lexical items. Data from the Switchboard corpus supports the latter prediction.