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Jared Rubin writes about the diverging history of the Christian and Muslim systems of finance back in the Middle Ages. At the time, both Christianity and Islam was enforcing restrictions on lending at interest; Islam also had a ban on financial speculation in general. At the same time, the Islamic world had a financial tool called the bill of exchange, which was meant to facilitate the movement of money between cities. When this tool spread into the Christian world, it became the basis for an explosion of international banking unlike anything seen in the world before then. Why? And why did it not have this effect in the Islamic world?

Bills of exchange worked like this.  Suppose you were a merchant traveling from Baghdad to Basra, to buy trade goods there. However, you don’t want to carry a great deal of money with you; to do so, you would need to hire bodyguards, not to mention the pack animals necessary to carry the precious metal itself (remember, silver and gold are quite heavy). This could be quite expensive; for example, the cost of moving gold bullion from Rome to Naples in this period has been estimated at between 8% and 12% of its value. So you deposit your money with an associate in Baghdad, and receive a bill of exchange for its value. This you must present to your Baghdad associate’s business partner in Basra, who will then give you cash. In this way, you don’t need to physically transport cash. And later, the business partners in Baghdad and Basra can settle their own balance, perhaps with a similar bill of exchange going the opposite direction.

Now, one can think of this transaction in two ways, which are not mutually exclusive. First, the business partners in Baghdad and Basra are clearly providing a service to you, the merchant, who can avoid the danger and expense of transporting your money. But remember too that you are fronting money to the issuer of the bill of exchange. Looked at this way, the bill issuer is borrowing your money.

Islamic law came down firmly on the side of the first understanding. The law required the merchant to pay a fee to the bill issuer—that is, the bill issuer is effectively being paid to borrow money. Furthermore, bills of exchange were dated, and needed to be redeemed by the specified date. If a bill were redeemed late, the merchant would be forced to pay a cumulative penalty. Unscrupulous businessmen sometimes exploited this by refusing to redeem bills of exchange on time, inflicting the penalty on the hapless merchant.

What this meant was that it was extremely risky to deal with bills of exchange from people you didn’t trust. Furthermore, you had little incentive to expand the network of people whose bills you used, since you were being forced to pay for the privilege. So bills of exchange, while useful in certain circumstances, did not stimulate the creation of the sprawling banking networks that grew in Europe later.

To understand the effect of the bill of exchange in Europe, we must understand the difference in conditions.

European trade was made particularly difficult because the different lands each had their own currencies. Furthermore, kings and princes often imposed bans on importing foreign coins into their lands. These bans existed for the good of the ruler alone: when the ruler issues his own currency, he earns seigniorage, the difference between the value of the silver or gold in a coin and its face value. So the more of his coins a ruler can impose on a captive populace, the more money he makes. (Worse, it was a depressingly common practice for rulers in need of cash to debase their currency, reducing its silver content so that each coin was worth less in reality.)

So to trade across lands, a European merchant needed a way to convert currencies—without paying the massive fees that local princes usually demanded. The bill of exchange answered the need. A merchant would deposit money with a banker in Florence, let’s say, and receive a bill of exchange payable in Lyons. The difference was that the bill of exchange was for a different currency than was deposited.

This was prohibited in Islamic lands, where currency swaps of this kind were viewed as speculation. But speculation was not banned in Christianity. So merchants were able to evade capital controls by creative use of debt contracts. Even better, merchants could take advantage of the predictable shifts of currency rates so that they would be repaid in more valuable money than they had lent out, effectively earning “stealth” interest and evading the Christian prohibition on usury.

Thus, Christian merchants and bankers had a huge incentive to expand their ties with other cities, since every additional city offered more opportunities to issue bills of exchange and therefore to lend money profitably. This is how the fabled Medici banking network was built up, for example: by establishing subsidiaries in cities across Europe and transacting bills of exchange between them, at considerable profit.

The upshot was that the creative use of bills of exchange supercharged international trade in Europe (while stunting intra-national trade, since there was no profit to be made in exchanging currencies), where its effects in Islamic lands had been more modest. And in Europe it led to the creation of the first international banking empires, where it did nothing of the sort in Islamic lands. All because of a few seemingly minor details: the Muslim prohibition of speculating in currencies, and Islamic law allowing the bill issuer to charge the lender instead of the other way around.

What lessons can we take away from this? First of all, the little things can have big effects. Second, there’s no way to predict the systemic outcome of a given tool, once human ingenuity gets turned loose to play. Third, the history of finance is pretty cool.