Why do central banks hold trillions of someone else's money?

The last of the four reasons FX exists is the strangest to explain and the largest in size per participant. Central banks collectively hold something on the order of $12 trillion in foreign exchange reserves — money that is, by construction, somebody else's liability.

What reserves are for

A reserve is a stockpile of foreign currency, mostly held as short-term foreign government debt rather than cash. Four jobs explain almost all of it:

• Paying for imports. A country that cannot produce its own energy or food must buy it in a currency it does not print.
• Servicing foreign-currency debt. Bonds issued in dollars must be repaid in dollars, whatever happens domestically.
• Managing the exchange rate. Reserves are the ammunition for intervening in the market.
• Crisis buffer. When foreign lenders stop rolling over their loans, reserves are what stands between a country and a sudden stop.

By currency, roughly 58% of allocated reserves sit in US dollars, around 20% in euros, with yen and sterling in mid single digits and the rest spread thinly. Those shares move slowly — over decades, not quarters.

Intervention, and what it looks like on a chart

When a central bank wants its currency stronger, it sells reserves and buys its own currency. When it wants it weaker, it does the reverse — and that direction is effectively unlimited, since it can create its own currency without limit. Supporting your own currency is the constrained direction: you can only sell reserves you actually have.

The clearest illustration is a peg. The Hong Kong dollar is kept inside a band of roughly 7.75 to 7.85 per US dollar. The monetary authority stands ready to trade at the band's edges, so the pair spends years wandering across a range of about 1.3%. The Saudi riyal sits near 3.75 to the dollar and barely moves at all.

Put a pegged pair and a free-floating one side by side and the difference is not subtle. A chart of USD/HKD looks like a flat line with texture; a chart of USD/ZAR looks like a mountain range. The flatness is not calm — it is a policy being enforced continuously by a very large buyer and seller.

Side by side has to mean on one scale, though. Give each pair its own axis and both come out looking eventful, because a chart fills the space it is given. Below, an authored float and the whole range of an authored peg are drawn on the same axis, which is the only version of the comparison that says anything:

A worked example of the mechanics

Suppose a country's currency is under pressure and its central bank wants to defend a level. It sells $5 billion of reserves and buys its own currency in the market.

• Reserves fall by $5 billion.
• The domestic currency in circulation falls by the local-currency equivalent.
• Demand appears in a market that had none.

If reserves are $50 billion, that single operation used a tenth of the stock. This is why market participants watch published reserve levels: they are a measurable, finite resource, and the arithmetic of how long they last is public. Note carefully what that is — an observation about a constraint, not a prediction about who wins.

The currency that got abolished

One final case worth carrying. The euro removed FX between its member economies outright — eleven of them in 1999, twenty-one today. Pairs like EUR/DEM and FRF/ITL simply stopped existing, and with them the hedging, the spreads and the exposures of every business trading across those borders. It is the largest natural experiment finance has ever run on the question what is a currency market actually costing us? — and it is why the composition of the FX universe is a political and institutional fact, not a permanent one.

Try it now

1. Two pairs over the same five years are below: one managed, one floating. Before reading the axis, guess which is which from the shape alone — then check the vertical scale each one needs.

2. Measure the full five-year range of the managed pair, top to bottom, and read the percentage. If it is under a couple of per cent across five years, you have measured a policy rather than a market — no floating currency stays that still by accident.
3. Press MAX on the floating pair and look at where the line begins. It does not start when the currency did. Coverage is a property of the archive, not of the currency, and confusing the two is how people conclude that a market is younger than it is.