The Order Protection Rule And Trade-Throughs

Imagine you're selling shares on one exchange at $50.01 while another exchange is showing a buyer willing to pay $50.05 for the same stock right now. If a trade prints at $50.01 anyway, the seller got a worse price than was available — that's called a trade-through, and it's exactly what the Order Protection Rule (Reg NMS Rule 611) exists to stop.

The rule requires every trading venue to avoid executing trades at a price worse than the best displayed, immediately accessible quote at any other venue — the protected quotation. In the US, every exchange's top-of-book bid and offer is protected. Before routing or filling an order, a venue has to check the consolidated tape and either match the best price, route the order to wherever that best price lives, or decline to trade through it.

This is why a retail order sent to one exchange can end up filled on a completely different exchange: the venue you routed to is legally required to find you the best protected price across the whole market, not just its own book.

There are specific, narrow exceptions. An intermarket sweep order lets a trader intentionally trade through a protected quote, provided they simultaneously route orders to execute against that better-priced quote elsewhere — the trader is taking responsibility for clearing the better price themselves. A flickering quote exception applies when the protected quote was displayed so briefly it couldn't reasonably have been accessed. And a venue experiencing technical problems can declare self-help, temporarily excusing other venues from routing orders to it.

Worked example. Suppose the NBBO is $50.00 bid / $50.05 offer, with the $50.05 offer displayed only on Exchange A. A trader wants to buy 1,000 shares and sends a marketable order to Exchange B, which is quoting $50.10. Exchange B cannot simply fill the order at $50.10 — that would trade through Exchange A's protected $50.05 offer. Instead, Exchange B must route the order (or the relevant portion) to Exchange A to get filled at $50.05, or reject/hold the marketable piece until it can be handled without trading through. This routing obligation is a core reason smart order routers exist: they're built to satisfy exactly this rule automatically.