The medieval banks of continental Europe facilitated trade by serving as payment intermediaries. Depositors commonly would pay one another by transferring bank balances with the aid of overdraft credit. We model this process in an environment of intermediate good exchange with incomplete contract enforcement. Our model suggests that the early banks were capable of accessing the "netting credit" that exists by virtue of there being a high proportion of offsetting transactions in an economy. Individual traders are unable to net their individual positions because of difficulty in enforcing contracts for future performance with the other traders. Banks, by standing between buyer and seller on a centralized basis, can internalize the offsetting nature of the whole set of trades. This original role of banks is still a vital one.