There are around 180 currencies in the world, but only a very small number of them play an outsized role in international trade, finance, and central bank foreign exchange reserves. In the modern era, the U.S. dollar has a dominant international presence, followed to a lesser extent by the euro and a handful of other currencies. Although the use of specific currencies is remarkably stable over time, with the status of dominant currencies remaining unchanged over decades, there have been decisive shifts in the international monetary system over long horizons. For example, the British pound only lost its dominant currency status in the 1930s, well after Britain stopped being the leading world economy. In a new study, we show that the currency that is used in international trade transactions is an active firm-level decision rather than something that is just fixed. This finding raises the question of what factors could augment or reduce the U.S. dollar’s dominance in world trade.