This paper deals with the construction of the concept of unit in the phonetic descriptions of French language during the 17th and 18th centuries. Increasing precision makes it necessary to regulate the number of units grammarians are able to identify. We propose an analysis of the different criteria which have been introduced during this period to solve the problem. From 1660, the question is brought under a new light by the authors of general grammars in connection with the problem of generality, as we try to show by comparing different classi¬fications of sounds proposed by the most important theoreticians of that time.