A stop order promises a trigger, not a price. A $50 sell stop that gaps to a $41 open still triggers, filling near $41 — 18 percent below the stop. The SEC's bulletin states it in one line: the stop price is a trigger that causes the stop order to become a market order, and the execution price can deviate significantly from it. The condition that decides how far is liquidity at the moment the market order arrives, which is exactly the condition that fails in the moves a stop is set for.

What the order actually does at the stop price

A sell stop sits below the current market and a buy stop sits above it. When the stop price is reached the resting instruction converts to a market order, and a market order is filled at the best available price rather than at any named one. The SEC's order-type bulletin makes the general point with a $3.00 stock: an order for 1,000 shares can fill 500 at $3.00 and the remaining 500 higher once the liquidity at the first price is used up. A stop order inherits that behavior in full.

FINRA's page on stop orders in volatile markets puts the same mechanic in the reader's terms: with a floor of $50 on a stock, the sell stop becomes a market order at $50, and if the market is moving fast the fill can be significantly less than $50 per share. The same page notes the outcome that annoys most: the stock can rebound and resume trading at its prior level after the position has already been sold.

Two further details change results and are set by the firm rather than by the order. Brokerage firms differ on whether a last-sale price or a quotation price triggers the stop, and the same bulletin warns that a short-term intraday move can produce an execution substantially worse than the stock's closing price that day. A stop-limit order removes the price risk and adds execution risk instead: the bulletin's example is a sell stop-limit with a stop at $3.00 and a limit at $2.50, which activates at $3.00 and then fills only at $2.50 or better. The trade-off between the two is worked through in the guide on stop and stop-limit orders.

Worked example: a gap through the stop

The order mechanics and the halt thresholds below are published; the price path is an example, because no single public quote illustrates a gap.

Hold 1,000 shares and set a sell stop at $50. The stock closes at $52 and, on news released overnight, opens at $41. There is no trade at $50 to fill against. The stop triggers on the first print and the market order executes near $41, producing $41,000 rather than the $50,000 the stop price suggests. The shortfall is $9,000, and the fill is 18 percent below the stop price, since $9 divided by $50 is 0.18.

Nothing in that sequence malfunctioned. The order did what the SEC bulletin says it does, and the guide on market and limit orders covers the same gap between a named price and a fill without the trigger attached.

The halts that interrupt the fill

Exchange-level circuit breakers run on top of the order. The SEC's glossary describes limit up-limit down bands set around the average price over the preceding five minutes: the bands are 5 percent, 10 percent, 20 percent, or the lesser of $0.15 and 75 percent, depending on the stock's price and whether it is a Tier 1 or Tier 2 NMS stock. If the price sits at a band for 15 seconds without moving back, trading pauses for five minutes, and the bands double during the last 25 minutes of the session for Tier 1 stocks and for Tier 2 stocks at $3.00 or below.

Market-wide halts use the S&P 500's single-day decline: 7 percent at Level 1, 13 percent at Level 2 and 20 percent at Level 3. A Level 1 or Level 2 trigger before 3:25 p.m. halts trading for 15 minutes; a Level 3 trigger closes the market for the rest of the day. A stop order triggered into a halted market waits, and reopening prices are where the fill is made. The published bands are the reason a fill can land far from the stop and still be an ordinary execution — the price difference the guide on a quote versus a fill takes apart in detail.

Source references

Primary sources used for this guide. Follow the links for the original scope, definitions and conditions.