Pre-industrial money supply typically consisted of multiple, often foreign currencies. Standard economic theory implies that this entails welfare loss due to transaction costs imposed by currency exchange. Through a study of novel data on Finnish nineteenth-century parish-level currency conditions, we show that individual currencies had principal areas of circulation, with extensive co-circulation restricted to the boundary regions in between. We show that trade networks, defined here through the regional co-movement of grain prices, proved crucial in determining the currency used. Market institutions and standard price mechanisms had an apparent role in the spread of different currencies and in determining the dominant currency in a given region. Our findings provide a caveat for the widely held assumption that associates multi-currency systems with negative trade externalities.