Why does trading sometimes stop?
On the worst market days, something counterintuitive happens: the market itself hits pause. Trading halts aren't failures — they're seatbelts, engineered after crashes taught their necessity.
Market-wide circuit breakers
US markets (echoed with variations worldwide) stop trading automatically when the S&P 500 falls too far in one day:
- Level 1: −7% → 15-minute halt;
- Level 2: −13% → another 15-minute halt;
- Level 3: −20% → trading closes for the day.
The design theory: panics feed on their own momentum; a forced pause lets humans read, machines recalibrate, and buyers find their nerve. March 2020 — the covid crash you studied in Course 1 — tripped Level 1 four times in two weeks: the seatbelts locked, the car did not crash through the rail.
Two details make the rule usable rather than trivia. The percentages are measured against the previous session's S&P 500 close, so the three trigger levels are known before the open every day; and the first two only fire before 3:25 in the afternoon New York time — a 7% fall in the last half hour is allowed to run, on the theory that a pause with no time left to reopen into helps nobody. Only Level 3 closes the day at any hour. The design dates from the October 1987 crash, when the Dow fell 22.6% in a single session with no pause at all; the modern S&P-based percentages replaced the original Dow-point thresholds in 2013.
Single-stock halts
Individual names pause too:
- Volatility halts — a stock moving too fast in minutes is stopped briefly to let the order book breathe (born from the 2010 "flash crash," when prices of great companies briefly printed absurd values in a cascade of automated selling).
- News-pending halts — a company requests a pause before releasing market-moving news, so nobody trades against information mid-publication.
- Regulatory halts — trading suspended while serious questions (fraud investigations, delisting) are resolved. These can last days and are a red flag by design.
What a halt means for you
Orders don't execute during a halt; they queue for the reopening, which often lands at a very different price — gap mechanics you already understand from Unit 2. If you ever watch a halted stock, remember: the pause is the system working, and the reopening print is the market's re-agreement, not a glitch.
Try it now
Read the current level of the broad US market off the right-hand edge of this chart:
- Take that number and compute what −7%, −13% and −20% of it would be. Drop a Level at each. Those three lines are the distances a single day would have to travel for the market-wide breakers to trip.
- Now Measure the worst single day in the year on screen and compare it with your three lines. How far off was it? Breakers exist for days rarer than anything on this chart.
- Then look for the other half of the story: a gap, where one session opens away from where the previous one closed. Press MAX and hunt for the biggest one you can find. That distance is price moving while nobody could trade — a halt does not stop the news, it only stops the trading.
- File the concept next to volatility and gaps: three faces of the same truth — prices are agreements, and agreements sometimes need a moment.