(LOO). For example, a market-on-open order is guaranteed to get the open price, whatever that may be. A buy limit-on-open order is filled if the open price is lower, not filled if the open price is higher, and may or may not be filled if the open price is the same. Regulation NMS (Reg NMS), which applies to U.S. stock exchanges, supports two types of IOC orders, one of which is Reg NMS compliant and will not be routed during an exchange sweep, and one that can be routed to other exchanges. Optimal order routing is a difficult problem that cannot be addressed with the usual perfect market paradigm. Liquidity needs to be modeled in a realistic way if we are to understand such issues as optimal order routing and placement. The Order Protection (or Trade Through) Rule (Rule 611) was designed to improve intermarket price priority for quotations that are immediately and automatically accessible, but its role in predatory trading behavior has faced mounting controversy in the recent years. Conditional orders A conditional order is any order other than a limit order which is executed only when a specific condition is satisfied. Stop orders A stop order or stop-loss order is an order to buy or sell a stock once the price of the stock reaches a specified price, known as the stop price. When the stop price is reached, a stop order becomes a market order. A buy-stop order is entered at a stop price above the current market price. Investors generally use a buy-stop order to limit a loss or to protect a profit on a stock that they have sold short. A sell-stop order is entered at a stop price below the current market price. Investors generally use a sell-stop order to limit a loss or to protect a profit on a stock that they own. When the stop price is reached, the stop order becomes a market order. This means the trade will definitely be executed, but not necessarily at or near the stop price, particularly when the order is placed into a fast-moving market, or if there is insufficient liquidity available relative to the size of the order. The use of stop orders is much more frequent for stocks and futures that trade on an exchange than those that trade in the over-the-counter (OTC) market. Sell-stop order A sell-stop order is an instruction to