How Currency Pegs Work, and Why They Break

A currency peg is a promise: this currency will trade at or near this rate against that one, and the central bank will make it so.

It is one of the few promises in finance where the party making it has a hard limit on its ability to keep it, and everyone can see what that limit is.

Why countries peg

Price stability for trade. If most of your imports and exports are invoiced in dollars, a stable rate against the dollar removes a large source of business uncertainty.

Imported credibility. A country with a history of high inflation can borrow the anti-inflation reputation of a larger economy by pegging to its currency, rather than building that credibility from scratch over decades.

Investment confidence. Foreign investors are more willing to commit capital when the currency risk is bounded.

The cost is monetary independence, and it is not a small cost.

The mechanism

Suppose a country pegs at 10 units to the dollar.

If market pressure would push it weaker — more people selling the local currency than buying — the central bank must buy its own currency, paying with foreign reserves. Every unit bought drains the reserve stock.

If pressure would push it stronger, the bank sells its own currency and accumulates reserves. It can create unlimited amounts of its own currency, so this direction has no hard limit.

That asymmetry is the whole story.

Direction	Resource used	Limit
Preventing appreciation	Own currency, created at will	Effectively none
Preventing depreciation	Foreign reserves	Finite and published

Defending against weakness is a countdown that everyone can watch.

The impossible trinity

Underneath every peg sits a constraint economists call the impossible trinity, and it is not a theory so much as an accounting identity with policy consequences.

A country can have at most two of these three:

1. A fixed exchange rate
2. Free capital movement
3. Independent monetary policy

Why all three cannot coexist: if capital moves freely and you fix the rate, then your interest rates must match whatever the peg requires. Set them lower than the anchor country and capital leaves, draining reserves. Set them higher and capital floods in.

The consequence is severe. A country pegged to the dollar with open capital markets must follow US interest rates — even into a domestic recession that calls for cuts. Its monetary policy is set abroad, for foreign conditions.

Countries resolve this differently: some accept the loss of independence, some impose capital controls, and most large economies simply float.

The pattern of a break

Peg collapses follow a recognisable sequence, and the individual episodes differ far less than their circumstances suggest.

1. Divergence. Domestic conditions drift from the anchor economy — higher inflation, a widening trade deficit, or a growth shock. The peg rate becomes increasingly disconnected from what the currency would otherwise be worth.

2. Pressure. Capital begins leaving. The central bank starts buying its own currency, and reserves decline.

3. Visibility. Reserve figures are published. Market participants calculate the runway.

4. Attack. Speculators short the currency. This is not villainy so much as arithmetic: if the peg is unsustainable, the trade has limited downside — the currency cannot rise much above a defended ceiling — and large upside when it breaks. The asymmetry attracts capital.

5. Escalation. The central bank raises interest rates sharply to make holding the currency attractive. This works against the currency and against the domestic economy simultaneously: high rates crush borrowers, businesses and mortgage holders. The defence itself causes a recession.

6. Capitulation. Reserves fall to a level where continuing is impossible, or the domestic damage becomes politically untenable. The peg is abandoned and the currency repricess sharply, often 20% to 50% within days.

7. Aftermath. Imports become expensive, inflation follows, and anyone who borrowed in foreign currency faces a debt whose real burden has jumped while their income has not.

Why the last point matters most

The most damaging feature of a peg break is rarely the exchange rate itself. It is foreign-currency borrowing.

A stable peg encourages businesses and governments to borrow in dollars, because dollar interest rates are lower and the exchange risk appears eliminated. When the peg breaks, that debt has to be repaid in a currency that just became far more expensive, out of revenue earned in one that just became far cheaper.

This is the mechanism that turns a currency event into a solvency crisis, and it is why the damage lands on companies that never traded a currency in their lives.

What makes a peg durable

Pegs that have lasted decades tend to share features:

• Reserves that are very large relative to the economy and to short-term external debt.
• A rules-based framework with automatic operation rather than discretion, which removes the question of political will.
• Domestic policy consistent with the peg rather than fighting it.
• A credible fallback, such as a band rather than a hard rate.

The common thread is that durable pegs are defended alongside fundamentals. The failures are attempts to defend one against fundamentals, where the peg substitutes for the adjustment rather than accompanying it.

What to watch

• Reserve levels and their rate of change, published monthly by most central banks.
• Short-term external debt relative to reserves.
• The gap between official and black-market rates, which is often the earliest honest signal.
• Forward market pricing, which reveals what the market expects the rate to be later.
• Domestic interest rates, since a sharp defensive rise signals strain.

The bottom line

A peg is a promise backed by a finite resource, made against opponents with no equivalent limit. Some hold for decades because the fundamentals and the framework support them. Those defended against economic reality end the same way, and the visible countdown on reserves is what invites the ending.

This article is educational and is not financial advice. Currency markets are volatile and leveraged trading carries a high risk of loss.