Order protection rule

The order protection rule, part of Regulation NMS - for National Market System - adopted by the Securities and Exchange Commission (SEC) in 2005, requires that every stock trading center establish and enforce a policy that ensures no transaction will be traded-through, or executed at a price that's lower than a protected quotation in that security displayed by another trading center.

A protected quotation is one that's immediately and automatically accessible. The order protection rule, also called Rule 611, does allow certain exceptions, which apply to limit orders, immediate-or-cancel (IOC) orders, and intermarket sweep orders (ISOs).