A market exists to let people trade, so a rule that stops trading looks strange at first. Yet almost every major exchange now has one. Circuit breakers and trading halts are the result of a long series of crises, each of which taught rule makers that a fast market also needs a way to pause.
Closing the doors: the first halts
For most of history, stopping a market meant shutting the building. The most dramatic case came at the outbreak of the First World War. In late July 1914, as investors across Europe rushed to sell securities for cash, exchanges closed one after another. Toronto and Montréal shut their doors, as did London and New York. The New York Stock Exchange stayed closed until December, and London did not reopen until January 1915.
The aim was to prevent forced selling from overwhelming the market and to protect the financial system during a moment of extreme uncertainty. Other closures followed in later decades, for example during the bank holiday declared in the United States in March 1933. These pauses were decisions made in the moment, not rules written in advance.
October 1987: the day that changed the rules
On Monday, 19 October 1987, stock markets around the world fell sharply. In New York the Dow Jones Industrial Average lost about 22.6 per cent in a single session, the largest one-day percentage fall in its history. Order volumes overwhelmed systems, specialists struggled to keep up and many investors could not get reliable prices.
The United States set up a presidential task force, often called the Brady Commission, to study what happened. Among its recommendations was a set of coordinated circuit breakers: pre-agreed points at which trading in stocks and related futures would pause. Exchanges introduced the first market-wide circuit breakers in 1988. For the first time, a pause was a planned feature of the market, with its triggers known in advance.
- ThresholdA broad index falls by a set percentage from the previous close.
- PauseTrading stops across marketplaces at the same moment.
- GatherParticipants review orders and information while nothing trades.
- ReopenTrading resumes on a schedule, often through a short auction.
How a market-wide circuit breaker works
The basic design is a ladder of thresholds measured against a broad index. In the United States the current levels, set in 2013, are declines of 7, 13 and 20 per cent in the S&P 500 from the previous day's close. Reaching the first or second level during most of the trading day causes a pause of 15 minutes. Reaching the third ends trading for the rest of the day.
The thresholds have been revised several times since 1988, because the original levels were expressed in index points and became too small as the index grew. Using percentages solved that problem. The idea behind each level is the same: give participants time to absorb information, check their orders and let the order book refill before trading resumes.
Single stocks and the sudden plunge of May 2010
On 6 May 2010, prices of many U.S. stocks and exchange-traded funds fell steeply and recovered within minutes. Some trades were executed at prices far from any reasonable value before the market steadied. The event showed that market-wide breakers were not enough: a single security could become disorderly while the overall index barely triggered anything.
Regulators responded with circuit breakers for individual stocks, introduced in 2010, and later with a system of price bands known as Limit Up-Limit Down, phased in from 2013. These bands stop trades from occurring outside a range around a recent reference price, and a stock that stays at the edge of its band is paused briefly.
The Canadian side
Canadian marketplaces apply their own halt rules, overseen by the national self-regulatory organization, which today is the Canadian Investment Regulatory Organization. Market-wide circuit breakers in Canada are coordinated with those in the United States, because many companies trade in both countries and a halt on one side of the border immediately affects the other. Canada also introduced its own circuit breakers for single securities in the years after 2010.
Separately, regulators can halt trading in a single company for reasons that have nothing to do with price swings, for example while important news is being released so that all investors receive it at the same time.
The trade-off behind every pause
Circuit breakers are a compromise. They interrupt the market's main job, matching orders, in exchange for time. In March 2020, the U.S. market-wide breaker was triggered four times in two weeks, and each pause ended on schedule with trading resuming in an orderly way.
Critics point out possible side effects. If traders see a threshold approaching, some may rush to trade before the pause, which could push prices toward the trigger. Others argue that halts move volatility to the moment of reopening instead of reducing it. Research on these questions continues, and the rules are adjusted as evidence accumulates. The broader lesson from a century of halts is that stopping is no longer an improvised reaction. It is part of how the matching engine is designed.
Educational content, not advice. This article describes historical events and how market mechanisms work. It is not financial advice and does not recommend buying, selling or holding any security.