The ERM Crisis and Black Wednesday

In 1990, the UK joined the Exchange Rate Mechanism (ERM), a system that pegged European currencies to each other within narrow bands ahead of the planned move to a single currency. The pound was pegged at a rate against the German mark that, within two years, most currency traders considered too high — the UK had higher inflation than Germany and a weaker economy, and a currency propped up above its natural level is exactly the kind of situation speculators look for.

Defending a peg that the market believes is wrong is expensive and, eventually, futile. To keep the pound from falling through its floor against the mark, the Bank of England had to buy pounds using its foreign currency reserves — but reserves are finite, while the market's capacity to sell a currency it thinks is overvalued is, for practical purposes, unlimited. George Soros's Quantum Fund, along with other speculators, built a large short position against the pound, betting that the UK would eventually be forced to devalue or abandon the peg rather than exhaust its reserves defending an unsustainable rate.

On September 16, 1992 — since remembered as Black Wednesday — the Bank of England tried raising interest rates twice in a single day, to 12% and then a promised 15%, in a desperate last attempt to make holding pounds attractive enough to stem the selling. It didn't work. By that evening, the UK announced it was suspending its ERM membership and letting the pound float, which it promptly did — falling sharply. Soros's fund is estimated to have made close to $1 billion on the trade, and the episode became so associated with him personally that headlines dubbed him "the man who broke the Bank of England."

The broader lesson quants take from Black Wednesday is about the mechanics of defending any fixed exchange rate: a peg is only as strong as the credibility of the central bank's willingness and ability to defend it, and once the market concludes a defense is unsustainable, the peg tends to break quickly and violently rather than slowly — a pattern that recurs in currency crisis after currency crisis.

The episode also reshaped UK monetary policy directly: the Bank of England moved to an independent, inflation-targeting framework in the years that followed, in large part as a response to the credibility damage from Black Wednesday.