In this contribution I am tracing the legal history of the concepts coutume and usage back from today’s international mercantile law to the Tribunal de la Conservation in early modern Lyon. From the late 19th century some theorists were regarding usage as normative when it could be derived from the consensus between contracting parties. We find this conception of usage, for example, in the CISG. On the other hand, the more romantical strain of theorists on the law merchant was stressing that customary law was normative regardless of the possibility to derive it from the parties’ agreements. In early modern Lyon merchants were invoking usages (and to a lesser extent also coutumes) at the Conservation frequently. Because of the juridification of this tribunal in the late 17th century, we expected that the use of the words coutume and usage was in line with the doctrinal conceptions of their days (according to which coutume was a form of written normative customary law and usage was a non-written normative customary law). This, however, was not always the case: sometimes the judges of the Conservation were using the words in a rather loose sense.