Order protection rule

A provision of the National Market System that forces trading centers to quote equivalent and consistent execution prices for a security on all exchanges that allow trading of that security. Also known as Rule 611 after its designation in the Federal Register, the Order Protection Rule is designed to prevent trade-throughs, or trades executed at prices other than the best-quoted price for that security. Rule 611 applies to all stocks on major indices and most over-the-counter stocks.