On October 27, 1997 the New York Stock Exchange shut early. The Dow had fallen 350 points, which tripped a circuit breaker, and after trading resumed it fell to 550 points down and the day ended half an hour ahead of schedule.
The detail that matters is what 350 points meant. It was a decline of 4.54%. The threshold had been set in points rather than percentages, and as the index rose over the following decade a fixed point trigger became progressively easier to hit. That single day is why the thresholds you see today are expressed as percentages.
Three Levels, One Reference Index
Market-wide circuit breakers halt trading across all US equity markets at once when the S&P 500 falls by a set percentage from the previous session’s close. The current framework, in force since February 4, 2013, has three levels:
The percentages are measured against the previous session's closing level of the index, not against the intraday high. A market that opens sharply higher and then falls 7% from that peak has not triggered anything, because the reference point is yesterday's close.
Levels 1 and 2 can each be triggered only once per day. A market that falls 7%, pauses, reopens and falls to 13% will halt again; one that recovers and falls back through 7% will not. Level 3 has no time restriction and no reopening.
Why the Thresholds Became Percentages
1997: a 350-point trigger that meant 4.54%
The SEC’s own study of that day records that the DJIA “declined 554.26 points (7.18%) to close at 7161.15”. The sequence was mechanical: the 350-point trigger was hit at 2:36 p.m., halting trading for thirty minutes; the market reopened at 3:06 p.m.; the 550-point level arrived at 3:30 p.m. and the session ended early.
What the regulator concluded
The 350-point trigger amounted to a 4.54% fall, and the SEC noted that this level had been reached on 11 previous one-day declines since 1945. A mechanism intended for extraordinary events had been calibrated to something that happened roughly once every five years, and it grew easier to hit with every point the index gained.
The thresholds were subsequently raised and re-expressed in percentage terms, which is self-correcting: 7% of the index means the same thing whether the S&P 500 sits at 1,000 or 6,000.
Note the reference index also changed. The 1997 breakers keyed off the Dow Jones Industrial Average, a 30-stock price-weighted index. Today’s key off the S&P 500, which is broader and capitalization-weighted, so a halt now reflects a decline across the market rather than in thirty large companies.
March 2020: Four Halts in Eight Trading Days
Between 9 and March 18, 2020, as the pandemic repriced global markets, the Level 1 breaker fired four times: on 9, 12, 16 and 18 March. Each came shortly after the opening bell, when overnight news met the first minutes of continuous trading.
Set against the record, this is the striking part: before 2020, the market-wide mechanism had halted US trading exactly once, on that day in 1997. In a single fortnight it did so four times.
None of those days reached Level 2. The 7% threshold was crossed repeatedly; 13% was not. The breakers behaved as designed, pausing trading for fifteen minutes and reopening into a market that continued to function.
The 3:25 p.m. Rule
Levels 1 and 2 stop applying at 3:25 p.m. ET, thirty-five minutes before the 4:00 p.m. close. A 7% decline at 3:30 p.m. produces no halt at all.
The reasoning is practical. A fifteen-minute halt late in the session would consume most of the remaining trading day and push a large volume of orders into a compressed reopening, which risks amplifying the disorder the halt exists to prevent. Level 3 keeps no such exemption: a 20% fall closes the market whenever it happens.
For anyone watching a sharp late-session decline, this is what it means. After 3:25 p.m. the only remaining brake is the 20% level, and between 7% and 20% the market simply trades.
Individual Stocks Use a Different Mechanism
A single stock collapsing does not trigger a market-wide halt. Individual securities are governed by limit up-limit down, which prevents trades outside a band around the recent average price rather than stopping trading outright.
The bands, and why they widen at the edges of the day
The SEC sets those bands at 5%, 10%, 20%, or the lesser of $0.15 or 75%, depending on the stock’s price and which tier it falls into. Lower-priced stocks get proportionally wider bands, because a few cents represents a larger percentage move.
The bands double during the opening and closing periods. Both are more volatile. The open absorbs everything that happened overnight, and the close carries the largest concentration of institutional order flow in the day, so a band calibrated for midday would halt normal trading at the edges.
This regime came out of the flash crash of May 6, 2010, when the Dow Jones Industrial Average fell nearly 1,000 points intraday and recovered most of the loss before the close, according to the joint SEC and CFTC report on the episode. Individual securities were the problem rather than the index: some traded at absurd prices for seconds at a time while the market-wide mechanism, keyed to a full-day decline, never came close to firing.
The SEC approved single-stock circuit breakers that June for S&P 500 constituents, extended them to all National Market System securities by June 2011, and then replaced them with limit up-limit down, which prevents bad prices rather than reacting to them after the fact.
What a Halt Does, and What It Cannot Do
A circuit breaker does not change what a company is worth. It interrupts the process of finding out.
The case for it is that a fifteen-minute pause lets information disseminate, gives market makers time to reassess, and breaks the feedback loop in which falling prices force automated selling that pushes prices lower. The case against is that it delays price discovery and can concentrate selling pressure into the reopening, since anyone who wanted out at 7% still wants out at 7:01.
- It does not prevent losses. Trading resumes, and the market can keep falling, as it repeatedly did in March 2020.
- It does not stop a fall in progress. A halt pauses execution, not the underlying repricing.
- Futures and overseas markets keep moving. A US equity halt does not freeze the instruments that price the same risk elsewhere.
- Level 3 is untested in the current framework. No 20% single-day decline has occurred since 2013.
During a halt, orders can usually still be entered, amended or cancelled, but nothing executes. If you hold a resting market order when a halt begins, it will fill at whatever price prevails on reopening, which may be far from where the halt started. A limit order will not.
Why the Reopening Is the Delicate Part
The halt itself is straightforward. Restarting is where the design work sits, because fifteen minutes of accumulated orders have to be matched at a single price without producing a second dislocation.
Exchanges reopen with an auction rather than by simply resuming continuous trading. Orders collect during the pause, the venue publishes indicative prices as the imbalance becomes visible, and the reopening price is the one that clears the largest volume.
This is the same mechanism used at the open and close of every ordinary session, applied to an extraordinary one.
Two consequences follow for anyone holding orders. The indicative prices published during a halt are informative but not binding, and they can move substantially in the final seconds before the auction runs. And the reopening price is set by the balance of orders that accumulated during the pause, not by the last trade before it, so the gap across a halt is frequently larger than the move that caused it.
What This Means for an Ordinary Investor
Almost nothing, and that is the point worth internalising, however alarming the coverage sounds at the time. A Level 1 halt is a fifteen-minute pause in a session running from 9:30 a.m. to 4:00 p.m. For anyone holding equities on a horizon measured in years, it is an event of no consequence whatever, however dramatic it looks on a screen.
The one group for whom halts matter is anyone trading intraday with leverage or stop orders, since a pause removes the ability to exit a position for a quarter of an hour while the risk keeps running. Everyone else is watching a plumbing event.
The practical precautions are narrow. Avoid resting market orders during volatile sessions, because a reopening price can be a long way from the last trade you saw. Expect quoted prices to be stale and brokerage platforms to be slow while a halt is in force.
And treat the halt itself as information about volatility rather than about the companies you own.
The Bottom Line
Circuit breakers are market plumbing rather than investor protection, and the distinction matters. They exist to keep an orderly market functioning through extreme moves, not to protect anyone from losses, and their history is largely a story of learning that the first calibration was wrong.
The 1997 halt, triggered by a 350-point move that represented 4.54%, is the reason the thresholds are percentages today. The four halts of March 2020 are the reason to know they exist. Between them, they show the mechanism working roughly as intended: a brief pause at 7%, no cascade to 13%, and a market that reopened and kept functioning each time.
For the session those halts interrupt, see stock market trading hours; for the venues that operate them, see the venues that operate them and market capitalization, which determines index membership.
