Queue Priority
Queue priority is the ranking system that determines the sequence in which resting limit orders at the same price level on an exchange order book are matched against incoming marketable orders, with the most common U.S. equity market convention being price-time priority — first ranked by best price and then, among orders at the same price, by time of arrival.
Every electronic limit order book must resolve the question of which resting order gets filled first when a marketable order arrives and multiple limit orders are posted at the same price. The answer is determined by the priority rules embedded in the exchange's matching algorithm. These rules have profound effects on market participant behavior because queue position directly determines the probability of execution at a given price level, driving firms to invest substantially in the infrastructure and strategies needed to achieve favorable queue position.
The dominant priority rule in U.S. equity markets is price-time priority, in which the best price always ranks first (a bid of $50.01 ranks ahead of a bid at $50.00) and among orders at the same price, earlier arrival takes precedence. The practical consequence is that the order first submitted at a given price level holds a privileged position and is unlikely to be displaced unless the market moves away from that price. Firms that submit orders with the lowest latency to an exchange gain queue position ahead of slower participants, which is why latency reduction is commercially valuable even for participants who are not engaged in pure speed arbitrage.
Alternative priority rules are used on some venues and in some market segments. Pro-rata priority allocates fills among competing orders at the same price level in proportion to their size, rather than in strict time order. This is the dominant priority rule in many interest rate futures markets, including CME Eurodollar and Treasury futures. Pro-rata rules reduce the premium on latency and favor larger displayed orders, but they can encourage quote spamming — the submission of very large orders with the intention of cancelling the unfilled portion after a partial execution.
Size-priority rules, which rank larger orders ahead of smaller ones at the same price, are rare but have been implemented in some markets to encourage aggressive displayed liquidity. Broker-priority rules, in which certain designated market makers or specialists retain first-fill rights, were historically important on the NYSE floor but are largely absent from modern electronic equity markets. Understanding which priority rule governs a specific venue is essential for market participants designing limit order strategies.