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Your Cambodian riel notes are probably great conversation pieces, the late King Norodom Sihanouk’s solemn visage glancing at the people to your left as you regale them with tales of your visit to Angkor Wat and Angkor Thom. But as international trading commodities, those notes don’t have quite as much utility. In fact, the overwhelming majority of currencies are of limited use outside their countries of issue. Even though there are 180 currencies in circulation throughout the world and majority of worldwide foreign exchange transactions involve a mere half-dozen of those currencies. It’s an extreme instance of the Pareto principle, with real-world applications. Here’s a look at those select currencies, and how they came to dominate the markets.

As any international traveler will confirm, the demand for United States Dollars across the globe is enormous and without serious competition. The cab drivers at the Ulan Bator airport in Mongolia will take only one non-Mongolian currency, and it’s neither the Russian ruble or the Chinese yuan. Thanks to a relatively stable government, a historically dynamic economy, and consistent value (low inflation) over time, the U.S. dollar serves as the de facto universal medium of exchange. Or to use the chosen term of art, the world’s primary reserve currency.