English auxiliary contraction has received much attention in the linguistic literature, but our knowledge of this variable has remained limited due to the absence of a thorough corpus study. This paper examines contraction of six auxiliaries in two corpora, considering three distinct phonological shapes in which they occur and the implications for an analysis of the grammatical processes that underlie the surface alternation in form. I argue that the data best support a two-stage analysis of contraction, one under which variation in the morphology is followed by phonetic and phonological processes. Moreover, I show that this particular analysis explains a number of patterns in the data that would otherwise be accidental. In this way, I underscore the importance of approaching the study of variable phenomena with both quantitative data and formal analysis.