Although it is evident in routine decision-making and a crucial vehicle of rationalization, commensuration as a general social process has been given little consideration by sociologists. This article defines commensuration as the comparison of different entities according to a common metric, notes commensuration's long history as an instrument of social thought, analyzes commensuration as a mode of power, and discusses the cognitive and political stakes inherent in calling something incommensurable. We provide a framework for future empirical study of commensuration and demonstrate how this analytic focus can inform established fields of sociological inquiry.