The topic of the lex mercatoria is of particular interest to the legal historian, both in relation to today’s new ideologies of internationalism, supported by a reference to the past, and with regard to the study of law sources. That being said, however, it is probably rather difficult to make a case for the existence in the Middle Ages and a substantial part of the modern era of an autonomous commercial system of rules and regulations, supranational, uniform and unfettered by local regulations. The existence of commercial conventions is undeniable. But they must be interpreted in a light that gives due consideration to the input of reciprocal influence, of the elaboration of doctrine and of the style of jurisprudence practised in the courts of the ancien régime.