Time Priority
Time priority is the secondary order matching rule in U.S. equity markets that governs the sequence of execution among limit orders posted at the same price level, awarding execution precedence to the order that was submitted to the exchange earliest in time, thereby rewarding market participants who are first to commit capital at a given price.
In a price-time priority order book — which is the standard structure for all U.S. equity exchanges — orders are ranked first by price and then, among all orders at the same price, by time of arrival. Time priority is the tie-breaker: when ten different firms have all posted bids at $50.00 and a marketable sell order arrives, the firm whose bid was submitted earliest fills first, the second-earliest next, and so on until the incoming order is fully executed or the price level is exhausted.
Time priority creates a powerful incentive to submit orders as early as possible. In U.S. equity markets, where trading in a given stock may attract hundreds of competing limit orders at the best bid or offer, queue position — the rank within the set of orders at the same price — is a primary determinant of fill probability. A limit order at the front of the queue has near-certain execution if the price is touched; an order at the back of a deep queue may go unfilled entirely if the price level is only partially consumed before the market moves away.
The commercial value of time priority in queue is the primary reason market-making firms invest heavily in low-latency trading infrastructure. Submitting a resting limit order one millisecond ahead of competitors can mean the difference between front-of-queue position that executes reliably and back-of-queue position that rarely fills. This queue competition also explains why HFT firms cancel and resubmit large numbers of orders: they lose time priority when they modify an order, so they must weigh the cost of giving up queue position against the cost of maintaining a stale resting order.
Time priority interacts with other priority rules at venues that layer in additional dimensions. Exchanges that offer reserve (iceberg) orders, which display only a partial size while hiding the full quantity, sometimes give reserve orders lower time priority than fully displayed orders to encourage transparent quote display. IEX's D-Limit order effectively allows certain orders to reprice without losing their time priority under specific conditions, a design feature that was contentious precisely because it modifies the standard time priority framework.