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The Order Protection Rule is Rule 611 of Regulation National Market System, adopted by the Securities and Exchange Commission in 2005. It prohibits trade-throughs: executing a trade at a price inferior to a better price displayed on another exchange. If the best bid for a stock is $50.05 on the New York Stock Exchange and your broker routes your order to a different exchange that fills it at $50.04, that is a trade-through, and it is prohibited. The rule's core purpose is to ensure investors always receive the best available displayed price.
Rule 611 was a direct response to a pre-2005 world where market fragmentation meant retail investors were regularly receiving worse prices than those publicly available elsewhere without knowing it.
The Order Protection Rule does not tell brokers exactly where to route orders. It tells exchanges and trading centers that they must have policies and procedures preventing trade-throughs. In practice, this requires trading venues to check the National Best Bid and Offer before executing a trade and to route orders away if a better protected price exists elsewhere.
The National Best Bid and Offer is the consolidated best bid and ask across all protected quotations at all exchanges at any given moment. When you place a market order, the routing logic downstream must honor that consolidated price. Executing at anything worse is a Rule 611 violation.
Not all quotations are protected under Rule 611. Only "protected quotations" qualify. These are displayed, immediately automated, and accessible at an exchange or the FINRA Alternative Display Facility. Manual quotations, dark pools, and over-the-counter venues are not protected, which is why so much trading activity moved away from displayed exchanges after 2005.
An important exception is the intermarket sweep order. A broker can execute against a protected quote at a venue and simultaneously route matching intermarket sweep orders to all better-priced protected quotes at other venues. This exception allows large institutional orders to execute across multiple venues quickly without triggering a trade-through violation on each leg.
Rule 611 changed U.S. equity markets fundamentally. Because any new exchange could gain immediate order flow by posting a better quote that brokers were legally required to honor, the number of exchanges multiplied. In 2005 when Regulation National Market System was adopted, eight exchanges traded equities. As of 2025, seventeen exchanges exist, with more planned for 2026.
Critics argue this venue proliferation fragmented liquidity, increased complexity for brokers, and created opportunities for latency arbitrage. High-frequency trading firms built strategies around exploiting the microsecond timing gaps between when better quotes appear and when order routing responds to them. This was not the intent of the rule but emerged as an unintended consequence.
The Securities and Exchange Commission hosted a public roundtable on Rule 611 on September 18, 2025 and a second on December 16, 2025 at the University of Austin. Securities and Exchange Commission Chair Paul Atkins, who voted against Rule 611 as a commissioner in 2005 and called it "unnecessary and anti-competitive" at the time, has indicated the commission will consider reform or repeal.
Supporters of the rule argue it protects retail investors by ensuring they receive best-price execution and acts as a backstop against brokers routing to inferior venues for payment for order flow reasons. Opponents argue best-execution obligations already require brokers to seek optimal prices, making Rule 611 redundant while adding structural distortions to the market.
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