Through the use of data collected among Slave Indians living in northern Canada, this paper explores a problem in ethnographic methodology: how to describe cultural rules such that contextual restrictions which operate upon them are identified and made explicit. Following a discussion of some of the ways in which the aims and assumptions of current sociolinguistic theory can be applied to this problem, a formal model is presented of Slave rules for travelling on the ice of frozen lakes and rivers. This model, which specifies the conditions under which a Slave hunter can be expected to cross an expanse of ice or avoid it, reveals the sensitivity of normative rules to variation in contextual features and illustrates both the value and feasibility of incorporating these features into ethnographic accounts.