This article discusses the use of private promissory notes in the sixteenth-century commercial metropolis of Antwerp. Students of financial history tend to look for first instances of financial techniques and institutions such as bills of exchange, share trading, sovereign debt and banks. However, financial innovation can also be found in the piecemeal adaptation of an older, existing technique, institution or instrument as the result of changes in the market and of demands exerted by particular groups within that economy. The outcome of this process is determined by the structure of the economy in question, its institutional arrangements and the willingness of authorities to adapt the rules.