Price-time priority in order matching is the rule almost every U.S. stock exchange uses to decide which order fills first: the order with the best price goes first, and when two orders share the same price, the one that arrived earlier wins. Price is the first sort key and it is absolute. Time is the tiebreaker. That two-step ranking runs the continuous order book throughout the regular trading day, and it decides where your order sits in line the moment it hits the exchange.
How Price Ranks the Book
Every exchange keeps an order book with two sides. Bids (buy orders) are sorted highest to lowest. Offers (sell orders) are sorted lowest to highest. The most competitive prices sit at the top on each side, and that top-of-book position is what determines who trades next.
A buyer willing to pay $50.10 sits ahead of a buyer at $50.05. A seller asking $50.12 sits ahead of one asking $50.15. No amount of waiting will let a less aggressive price jump ahead of a more aggressive one. If you want to move up the queue, you improve your price. That’s the only way.
The distance between the best bid and the best offer is the spread, and it is the cost of crossing from one side of the market to the other. The whole design of price-time priority rewards whoever posts the most competitive price with the first fill, which is what keeps spreads tight.
Time as the Tiebreaker
When two orders rest at the same price, the exchange breaks the tie with timestamps. An order that arrived at 10:00:01.000 fills before an order that arrived at 10:00:01.001. Exchange servers record these timestamps down to the microsecond or nanosecond, and the sequence is strict. There are no exceptions for order size, account type, or trading volume. A retail investor who posted a limit order five seconds before a large institution at the same price fills first.
This first-in-first-out discipline is what makes the system feel fair to most participants. Everyone can see where they stand in the queue. The tradeoff is that it heavily rewards speed, which is why professional firms invest so much in shaving latency off their order submissions.
How the Matching Engine Walks the Queue
The matching engine is the software that pairs incoming aggressive orders against resting orders in the book. When a market order or a marketable limit order arrives, the engine walks through the queue in strict price-time sequence: it fills against the order at the top of the book first, then the second, then the third, and so on until the incoming order is satisfied or the liquidity at that price is exhausted.
Partial fills are common. If the first resting order in line has 200 shares and the incoming order wants 500, the engine fills those 200, marks that resting order complete, and immediately moves to the next order in the time queue at the same price. If the entire price level only holds 400 shares, the engine moves to the next price level and continues filling there. Every share is allocated by the queue, with no discretion and no manual intervention.
This process happens in microseconds. On a busy day, a single exchange might process millions of matches, each one following the same deterministic path through the book. The consistency is the point. You can predict exactly how your order will be treated before you submit it.
Displayed and Hidden Orders in Line
Not every order in the book is visible. Traders can submit hidden (non-displayed) orders, or reserve orders that show only a fraction of their total size. Hidden orders still participate in the queue, but they sit behind all displayed orders at the same price. A displayed bid at $50.10 that arrived at noon will fill before a hidden bid at $50.10 that arrived at 11:00 AM, even though the hidden order has been waiting longer.1Cboe. Hide-and-Seek: Hidden Liquidity on U.S. Exchanges
So the priority hierarchy actually runs in three tiers: price first, then display status, then time. Exchanges enforce this because displayed orders contribute to price transparency and help other participants make informed decisions. Hiding an order is a legitimate tactic for avoiding information leakage on a large position. The cost is lower execution priority.
Reserve (iceberg) orders split the difference. The displayed portion competes with other visible orders at that price. The hidden reserve portion drops to the non-displayed priority tier. When the displayed portion fills, the exchange automatically replenishes it from the reserve, but the replenished portion typically receives a new timestamp. Fully hidden orders sit behind even the reserve portions on most exchanges.
What Changing Your Order Does to Your Place in Line
Changing the price of a resting order destroys your time priority. Exchanges treat a price change as a cancellation of the old order and the submission of a new one. You get a fresh timestamp and go to the back of the line at the new price level. There is no workaround. If you entered a bid at $50.10 with an early timestamp and change it to $50.11, you start over in the $50.11 queue behind everyone already there.
Reducing your order’s size is more forgiving. Most exchanges let you trim shares while keeping your original timestamp for the remaining quantity. If you posted 1,000 shares at $50.10 and reduce to 500, those 500 shares keep their place in line. Increasing size works differently. It counts as adding new liquidity, and the exchange assigns a new timestamp to the added shares, or in some cases to the whole order, pushing you to the back.
These rules exist to prevent gaming. Without them, a trader could submit a large order to claim a queue position, then immediately shrink it to a trivial size while holding that prime spot. Letting you pull back without penalty but not expand without rejoining the line strikes a reasonable balance.
Tick Sizes and How Long the Queue Gets
The minimum price increment, or tick size, determines how finely the price queue can be sliced. Under Rule 612 of Regulation NMS, the SEC amended the tick size regime to create two tiers for stocks priced at $1.00 or more. Stocks with a Time Weighted Average Quoted Spread (TWAQS) of $0.015 or less trade in $0.005 increments. Stocks with a wider spread trade in the traditional $0.01 increments.2U.S. Securities and Exchange Commission. Tick Sizes – A Small Entity Compliance Guide
Tick size has a direct effect on queue dynamics. A smaller tick creates more price levels, which means the queue at any single price level is shorter. That makes time priority less valuable, because a trader can step ahead of the entire queue by improving the price by half a cent instead of a full cent. With a $0.01 tick, the cost of jumping the queue is higher, so traders invest more effort in securing early timestamps.
The SEC evaluates TWAQS twice a year. The first evaluation period runs January through March and sets the tick size from the first business day of May through the last business day of October. The second evaluation period runs July through September and sets it from the first business day of November through the last business day of April. A stock’s tick size can change every six months based on its recent trading characteristics.2U.S. Securities and Exchange Commission. Tick Sizes – A Small Entity Compliance Guide
Odd-Lot Orders Sit Differently
An odd lot is any order for fewer than 100 shares (or whatever the round-lot size is for a given stock). Historically, odd-lot quotes were invisible to the broader market and had no impact on the National Best Bid and Offer (NBBO). A trader could post the most aggressive bid in the market, but if it was for 37 shares instead of 100, it wouldn’t appear in the published NBBO.
Starting April 27, 2026, the Securities Information Processors (SIPs) will begin disseminating top-of-book odd-lot quotations, including a new data element called the Best Odd Lot Order (BOLO), which aggregates the best odd-lot bid and offer priced better than the NBBO across all exchanges.3Consolidated Tape System (CTS) / Consolidated Quote System (CQS). SIP Odd Lot Quotes and Best Odd Lot Order (BOLO) Implementation: Frequently Asked Questions That increases transparency, but odd-lot quotes are still not “protected” under Regulation NMS. An exchange is not required to route to or honor an odd-lot quote the way it must honor a round-lot protected quotation. Odd-lot orders participate in price-time priority within their home exchange’s matching engine. They just lack the cross-exchange protections that round-lot quotes receive.
Price Priority Across Exchanges: The Trade-Through Rule
Price-time priority governs the queue within a single exchange, but the U.S. equity market is fragmented across more than a dozen venues. Rule 611 of Regulation NMS, often called the Order Protection Rule or the trade-through rule, extends price priority across exchanges. It requires every trading center to maintain policies and procedures reasonably designed to prevent executing a trade at a price worse than a protected quotation displayed on another exchange.4eCFR. 17 CFR 242.611 – Order Protection Rule
If NYSE is displaying the best bid at $50.10 and Nasdaq has a bid at $50.09, a seller on Nasdaq cannot simply execute against the $50.09 bid when a better price exists at NYSE. The trade needs to route to NYSE first, or use an intermarket sweep order that simultaneously sends orders to all exchanges with better prices to clear them before executing locally.
One important limit: Rule 611 enforces price priority across venues but does not enforce time priority across venues. Two exchanges might both display a bid at $50.10, and the rule doesn’t dictate which exchange’s bid fills first. That decision depends on where the seller’s order routes, which is driven by broker routing logic, fee structures, and speed of execution. There is no consolidated national queue.
The rule also includes exceptions for situations where strict enforcement would be impractical, such as when the exchange displaying the better price is experiencing a systems failure, during single-price opening or closing auctions, and for certain benchmark or contingent trades where the price isn’t determined by the quote at the time of execution.4eCFR. 17 CFR 242.611 – Order Protection Rule
Where Price-Time Priority Doesn’t Apply
Two important situations don’t follow price-time priority, and knowing the boundary matters if you trade across products or around the open and close.
U.S. Options Exchanges
Most U.S. options exchanges use pro-rata allocation instead. Price still comes first, but time priority is replaced by size priority. When an incoming order matches against multiple resting orders at the same price, each resting order receives a share of the fill proportional to its size relative to the total liquidity at that price level.5Eurex. What Actually Is Pro Rata Matching?
A concrete example from NYSE’s documentation: three sell orders sit at the best offer of $10.00, sized at 50, 40, and 10 contracts. A buy order for 20 contracts arrives. Under price-time priority, the earliest order would fill all 20 contracts. Under pro-rata, the 50-contract order gets 10 contracts (50% of available liquidity), the 40-contract order gets 8, and the 10-contract order gets 2.6NYSE. U.S. Equity Options Market Models When the math produces fractional contracts, the remainders are typically allocated by time priority.
The list of pro-rata or hybrid options venues includes NYSE American, Cboe, Cboe EDGX, Cboe C2, Nasdaq ISE, Nasdaq GEMX, Nasdaq MRX, Nasdaq PHLX, and MIAX.6NYSE. U.S. Equity Options Market Models The logic is that options markets have wider spreads and less frequent price changes. Pure time priority would let a single fast participant camp at the best price and capture nearly all the flow, discouraging other market makers from quoting. Pro-rata rewards market makers for posting larger size instead.
Opening and Closing Auctions
Price-time priority operates during continuous trading, but the trading day begins and ends with auctions that follow different rules. In an opening auction, orders accumulate over a pre-market collection period and then execute simultaneously at a single calculated price that maximizes the number of shares traded. There is no time queue. All orders at or better than the clearing price fill at the same price, regardless of when they were submitted during the collection window.
Closing auctions work the same way and have become increasingly important as more assets are benchmarked to closing prices. Both auction types use specialized order types (market-on-open, limit-on-open, market-on-close, limit-on-close) that receive priority in the auction match over standard limit and market orders. Pooling all interest into a single clearing event concentrates liquidity and reduces the price impact that large orders would have during continuous trading.
The practical takeaway: an early timestamp is valuable during continuous trading and irrelevant in an auction. In the auction match, only your price and your size determine your fill.