Historical linguistics is at times akin to archaeology. The researcher pieces together bits of evidence to create a complete picture of a particular phenomenon. For a linguist studying a signed language, the task is more daunting still, due to the relative dearth of available data. Nonetheless, linguistic typology and a thorough understanding of the language under investigation can yield exciting returns. This article explores the development of the negative modal can�t in ASL. This study involves many overlapping areas, including modality, negation, lexicalization, and grammaticization. Because of the historical relationship between ASL and French Sign Language (LSF), a diachronic study of certain grammatical features of ASL necessitates a discussion of Old LSF (see Lane [1984] for an account of the historical relationship between ASL and LSF and of the circumstances that brought LSF to the United States). Finally, one cannot ignore the sociolinguistics of the Deaf communities in France and North America. The result is a holistic investigation of the development of can�t that suggests that this modern ASL sign developed not from a positive modal expressing possibility or ability, as one might expect, but from a modal indicating deontic necessity, specifically, Old LSF il faut �it is necessary.�