Electronic stop-loss orders and protected price bands

What a stop-loss order waits for

A stop-loss order is a resting instruction that becomes executable only after the market reaches a stated trigger price. The trigger is the instant the stated stop price becomes part of the quoted market and the order is released for matching.

When the trigger became a market order

Once electronic matching replaced open-outcry handling, a triggered stop-loss was treated as a market order and no longer received discretionary fill management from a floor broker. A market order is an instruction to trade immediately against the next available opposite-side interest, with no trader-chosen limit on the print.

Electronic futures venues later added price protections so a triggered stop-loss would not be filled at extreme unintended prices the way an unconstrained market order could.

The other side of the book

A triggered buy-stop still fills only if matching software finds a resting sell-limit willing to take the other side. Reaching the stop price does not complete the trade by itself. A limit order is a resting bid or offer that names the worst acceptable price and supplies the other side of a triggered stop.

Slippage and the protected price band

Scheduled news can move grain futures 20 to 30 cents almost instantly and crude oil by a dollar or two, conditions in which a working stop-loss is not guaranteed to exit at the stated stop price. Slippage is the gap between the intended stop price and the eventual fill when the book is thin or prices jump.

If a protected stop-loss is filled, the fill is constrained to an exchange-defined reasonable-price range that varies by market and is updated as conditions change. That protected range is an exchange-calculated band around the bid and offer at trigger time inside which a filled stop is allowed to print.

For the e-mini S&P, the protection point was 3.00, so a triggered fill could not exceed 3.00 above the best ask or 3.00 below the best bid, a 6.00-point protected price band. The protected price band is the full interval spanning the buy-side and sell-side protection points around the quote at the moment of trigger.

Under ordinary trading conditions those price protections typically remain unused and unnoticed.