Intermarket Sweep Orders

The Order Protection Rule normally forbids trading at a price worse than the best quote displayed anywhere else. That's usually what a trader wants — but not always. If a trader needs to buy a large block of stock right now, waiting for a router to walk through every venue one at a time, in strict price order, can be too slow: prices move, and by the time the router reaches the third venue the quote there may have changed. The intermarket sweep order (ISO) exists for exactly this situation.

An ISO is marked with a flag telling the receiving exchange: "I am aware there may be better prices on other venues, and I am taking responsibility for accessing them myself — fill me here anyway." To use the flag legitimately, the trader (or their broker) must simultaneously route separate orders directly to every other venue displaying a better protected price, sized to clear those quotes. Because the trader has independently satisfied the intent of the Order Protection Rule — no better-priced quote is left untouched — the receiving venue is allowed to execute immediately without checking or routing on the trader's behalf.

This matters most for large or urgent orders. A single marketable order routed through the normal protection-rule waterfall visits venues sequentially, and each hop costs time; by the time it reaches later venues, the market may have moved, leaving the trader worse off than if all venues had been hit at once. A trader who fires a coordinated set of ISOs to every venue simultaneously trades faster and with more certainty about the total fill, at the cost of taking on the operational responsibility of making sure every better-priced quote really does get cleared.

Worked example. The NBBO offer is $50.05, but that price is only available for 200 shares on Exchange A; Exchange B is offering $50.06 for 1,000 shares. A trader who needs to buy 1,000 shares immediately sends two simultaneous ISOs: one for 200 shares to Exchange A at $50.05, and one for 1,000 shares to Exchange B at $50.06. Because the order to Exchange A clears the better-priced quote at the same moment, Exchange B is permitted to fill the ISO immediately at $50.06 without first routing anything itself, even though $50.06 is technically worse than $50.05.