  Second, it permitted the development of international currency exchanges, which place the exchange rate of a country's currency largely outside the control of that country's government. A monarch can establish a currency, and stipulate how much of some metal each unit of that currency shall contain. However, monarchs cannot control the rates of exchange between their currencies and those of other countries. These rates depend on the relative scarcity of money in the countries in question, and they are "fixed by the general opinion of the merchants, never by the decrees of the prince" (SL 22.10). For this reason "the exchange of all places constantly tends to a certain proportion, and that in the very nature of things" (SL 22.10).