A Korean steelmaker sells to a buyer in Brazil. Neither firm is American, and neither earns dollars at home. The invoice is written in dollars anyway. Exchange rates explain how won and reais get converted. They do not explain why a third country's money sits in the middle.
What a Reserve Currency Actually Does
A reserve currency is money people outside the issuing country hold by choice, not because a law requires it. The dollar does four jobs.
Central banks hold it. Countries keep foreign money to pay import bills and steady their own currency in a panic. The Federal Reserve's 2025 review put it at 58 percent of disclosed official reserves in 2024, down from 72 percent in 2001.
Exporters invoice in it. A contract must name a currency, and outside Europe it is usually the dollar. Averaged over 1999 to 2019, the same review found dollar invoicing on 96 percent of trade in the Americas and 74 percent in Asia and the Pacific.
Borrowers owe in it. Governments and firms outside the United States issue bonds and take loans in dollars. Bank for International Settlements data show many trillions of dollars of credit owed by borrowers outside the United States who are not banks.
Traders route through it. To turn Thai baht into Mexican pesos, a bank sells baht for dollars, then buys pesos. Two trades rather than one, and still cheaper, because the direct baht market for pesos is thin. That vehicle role put the dollar on one side of 89.2 percent of foreign exchange trades in the BIS survey of April 2025.
Why Oil Trades in Dollars
Crude benchmarks are quoted in dollars a barrel. The popular explanation is a secret 1974 bargain in which Saudi Arabia agreed to sell oil only for dollars in return for American protection. The documents do not support it. The two countries signed a broad cooperation agreement in June 1974, and a separate confidential arrangement had the Saudis buy Treasury securities outside the normal auctions. Neither set the currency oil is priced in.
The duller explanation fits better. Buyers compare one cargo against another, so one quoting currency makes prices comparable, and the dollar already had the deepest financing markets attached. The convention followed the plumbing.
How Sterling Handed the Job Over
Before 1914 the pound did all four jobs. Britain ran the largest trade network, London's banks turned trade bills into cash on demand, and the United States had no central bank until the Federal Reserve was created in 1913.
The handover was messier than the textbook version. Barry Eichengreen and Marc Flandreau, using reserve data they assembled themselves, found the dollar had passed sterling by the mid-1920s, two decades earlier than the standard account, and that the two traded places again afterwards.
Bretton Woods made it formal in 1944: forty-four countries fixed their currencies to the dollar, and the United States agreed to exchange dollars for gold at $35 an ounce.
The Gold Window Closed and the Dollar Stayed
In 1960 the economist Robert Triffin named a flaw the system could not fix. The world needed a growing supply of dollars, and only American deficits could supply them. The more dollars sat abroad, the less believable the promise to redeem them all at $35. Supply the world or defend the peg, but not both forever.
He was right. On August 15, 1971, President Nixon suspended conversion of dollars into gold for foreign governments, and fixed rates collapsed within two years. The obvious prediction was that the dollar would lose its position.
Foreign use of dollars kept growing instead. Whatever the world wanted dollars for, it was not the gold.
Why Nobody Switches First
The explanation is circular, and that is the mechanism. Dollar markets are deep because everyone uses them, and everyone uses them because they are deep. A bank quoting baht against pesos directly would face wider spreads and fewer counterparties than it gets by routing through dollars, so the direct market stays thin.
One piece is not circular. A reserve currency needs a safe asset to sit in, and that asset has to be sellable in size, at short notice, without the sale moving the price. Treasury securities do that better than anything else, mostly because there are so many of them. The Treasury market is the largest and most heavily traded government bond market in the world. A central bank can move billions in an afternoon; a smaller bond market would move the price against the seller. Nobody decreed this, and it is the hardest thing to copy.
What the Position Is Worth to the United States
Foreign demand for Treasury securities is demand the government does not have to bid for at home, which means a lower yield. Treasury yields anchor mortgage and corporate rates, so cheaper government borrowing reaches ordinary Americans too.
Valéry Giscard d'Estaing, France's finance minister in the 1960s, called this an exorbitant privilege. His complaint went past cheap credit. The United States borrows from the world in its own currency, through low-yielding safe assets, and invests abroad in higher-yielding ones, so it earns more on what it owns overseas than it pays on what it owes.
The Bill the United States Pays
Money coming in to buy dollar assets has to buy dollars first. That demand comes from portfolios rather than trade, and it holds the currency higher than trade alone would. An expensive dollar makes American exports dearer abroad and imports cheaper at home. The cost lands on a narrow group, factories and farms and anyone selling against imports, while cheaper imported goods spread thinly over everyone.
Triffin's dilemma survived the gold peg by changing shape. Demand for safe dollar assets grows with world output, which grows faster than the American economy. Meeting it means US external liabilities keep growing relative to the economy standing behind them. Refuse, and the world is short of safe assets. Supply them forever, and someone eventually asks whether they are still safe.
What It Costs Everyone Else
Borrow in dollars while earning your own currency, and a rising dollar raises your repayments in real terms though nothing in your business changed. A Turkish firm owing $10 million sees the lira cost of that debt rise about 25 percent if the lira falls 20 percent against the dollar. Emerging market borrowers hold a great deal of this debt.
The Federal Reserve sets policy for the American economy, but the effects do not stay there. When the Fed raises rates, dollar funding costs more everywhere, the dollar tends to strengthen, and money leaves countries that had no part in US inflation. Their central banks often tighten in response, into economies that did not need it.
Sanctions are the third channel. Cross-border dollar payments generally settle through correspondent accounts in the United States, which puts them under American law. Cutting a foreign bank off from dollar clearing needs no cooperation from its own government. The broader lesson came in February 2022, when the United States and its partners immobilized roughly $300 billion of Russian central bank reserves. Most of those assets sat in Europe rather than America. Reserves held inside someone else's financial system are held on that system's terms.
The Challengers, and Why the Numbers Move Slowly
The euro has the scale and has been available since 1999, but its share of disclosed reserves peaked near 28 percent in 2009 and has not returned there. The euro area issues no single government bond. German, Italian and French bonds are priced apart because their credit risk differs, so nothing matches the depth of the Treasury market.
China is the world's largest exporter, yet the renminbi sat near 2 percent of allocated reserves through 2025 in IMF data. Capital controls are the binding constraint. A reserve asset has to be something you can sell and take home whenever you choose, and China limits money crossing its border on purpose, to protect its exchange rate and its credit system. Loosening those controls enough to satisfy reserve managers means giving up the control they exist to provide.
Gold is doing something real. Central banks bought more than 1,000 tonnes in each of 2022, 2023 and 2024, on World Gold Council figures, and gold held at home cannot be frozen from abroad. It pays no interest, settles slowly, and will not buy a cargo of wheat.
Central bank digital currencies and rival payment networks change the plumbing rather than the denomination. Moving money faster between countries is a different problem from persuading the world to invoice, borrow and save in something new.
The share does move. On the Fed's numbers the dollar's reserve share fell roughly fourteen points between 2001 and 2024, a real decline and not a collapse. Confident predictions of imminent replacement have appeared in nearly every decade since 1971, and none has landed yet. That is not proof that none ever will. It is a reason to distrust anyone offering you the date.
