They were a safe way to move credit over long distances, and could be secured against fraud with seals and code words. They allowed Christian merchants to get around the Roman Church’s strict prohibition on usury—for when money was changed between currencies, exchange rates could be manipulated artificially in the lender’s favor, effectively allowing profit to be baked into the trade without officially defining this as interest. Better yet, bills of exchange could also be traded and circulated—sold at a discount to third parties who could go on to cash them themselves.
