Recent research in sociolinguistics has demonstrated the need for looking at language in a dynamic framework, that is, for not imposing the traditional synchronic-diachronic dichotomy on linguistic studies. Support for the dynamic framework has been given from various oral languages. This paper attempts to test variation theory with historically related visual languages, French Sign Language (FSL) and American Sign Language (ASL), by examining an historical change occurring in both languages, the change of two-handed signs on the face to one-handed signs. Data were collected from 135 deaf informants, 75 ASL signers and 60 FSL signers. Informants were selected on the basis of geographic location, ethnic origin, and sex. The results of this study support viewing languages in a dynamic framework, since traditional static theories have no formal mechanism to handle the inter- and intralingual variation that occurred in this study. The change from two-handed to one-handed signs patterned implicationally (in similar orders) for French and American signers. However, FSL is undergoing the change significantly more slowly than ASL. Older signers in both languages retain more of the older two-handed forms than younger signers. Within the U.S., Black Southern signers preserve more of the two-handed signs as compared with other U.S. signers. (Author/DB)