What role did foreign exchange play in the in the strategies of merchants from Rouen? Was it a mere instrument for payment or credit, or was it a means of increasing profits? To answer these questions, the inquiry relies on two types of in-depth case studies on both the models provided by the accounting handbooks, and the business transactions of some great merchants from Rouen operating between Seville and Hamburg on the Atlantic side, and, with regard to inland areas, as far as Lyon and Frankfurt. The results clearly show how skillful the businessmen of Rouen had become in mastering the foreign exchange practice. As well as their Southern competitors, they could understand the intricacies of the “cambio per arte” and play successfully on the exchange margins, something that allows for a challenging of the cliché regarding the lagging French trader before the eighteenth century. Thus, while giving priority to the commodities trade, the businessmen of Rouen knew perfectly well to what extent a proper use of foreign exchange could maximize their profits or reduce their losses. Comparing these individual strategies, it becomes clear that they were using the same channels. This underlines the systemic nature of the functional relationships that tied Rouen, Seville and Antwerp particularly, in the context of European commercial flows.