Time Priority and Price Priority: Who Gets Filled First

Key Takeaways

• Time priority and price priority are the two keys: a better price always ranks ahead, and within a price level, the earliest-arriving order fills first regardless of size.
• Modifying an order or increasing its size resets the timestamp on most venues, sending it to the back of the queue and potentially costing the fill.
• Investors frequently assume the iceberg reserve retains queue position after the displayed slice fills, but each replenishment gets a fresh timestamp and starts at the back.
• Queue position is a measurable asset for market makers; arriving even one microsecond earlier at a popular price level can determine whether a fill is captured or missed.

What It Is

Every limit order book applies a deterministic ranking to resting orders. The two primary keys are:

• Price priority. A more aggressive price ranks ahead of a less aggressive one. On the bid, higher prices rank first. On the ask, lower prices rank first.
• Time priority. Among orders sharing a price, the order that arrived first at that price ranks first. Each later order joins the back of the queue.

This combined "price-time" rule is the default on Nasdaq, NYSE Arca, NYSE Pillar, Cboe BZX, EDGX, and most major venues. Some venues add a third key (display priority, customer priority, or pro-rata allocation) that interacts with time priority in specific ways.

The Intuition

If you post a buy at 50.00 and ten orders arrive after yours at the same price, you should fill before any of them when sellers cross the spread. That is the contract the matching engine offers in exchange for displaying liquidity early. Without time priority, there would be no incentive to be the first to post a quote, since latecomers could free-ride on your price discovery work.

Price priority is more obvious: a buyer willing to pay 50.01 should be served before a buyer offering only 50.00. That keeps the book economically rational.

How It Works

When a marketable order arrives, the matching engine sweeps through resting orders in priority order until the marketable order is filled or no more eligible orders remain. The standard sequence at most US lit equity venues is:

1. Best price level on the opposite side
2. Within that price level, oldest timestamp first
3. Then move to the next price level if size remains

Several refinements modify the basic rule:

• Display priority. Many venues rank displayed (visible) orders ahead of non-displayed orders at the same price. A hidden order at 50.00 sits behind every displayed 50.00 order regardless of timestamp.
• Reserve refresh. When an iceberg's displayed slice fills, the new replenishment slice gets a new timestamp and goes to the back of the queue at that price.
• Cboe BZX customer priority. On options exchanges and some equity venues, retail customer orders rank ahead of professional or broker-dealer orders at the same price.
• Pro-rata allocation. Used on some futures and options venues (CME, certain CBOE products), pro-rata fills allocate by size rather than time. Most cash equity markets do not use pro-rata.

Crucially, price improvement breaks priority. If you cancel and repost one cent better, you jump to a new (better) price level and start a new queue there at the front.

Worked Example

The book at AAPL on a single venue looks like this on the bid side at 9:45:00.000 ET:

Bid 200.00
  Order A  500 sh   posted 09:30:01.123
  Order B  200 sh   posted 09:31:44.502
  Order C  900 sh   posted 09:33:12.011  (iceberg, displays 200, reserve 700)
  Order D  300 sh   posted 09:40:00.044

A market sell of 1,000 shares arrives. The engine fills in priority order:

Fill 500 from Order A   (timestamp wins)
Fill 200 from Order B
Fill 200 displayed slice from Order C
Fill 100 from Order D   (next in time at 200.00)

After the fill, Order C's reserve refreshes a new 200-share slice at 200.00 with a new timestamp of approximately 09:45:00, placing it behind Order D's remaining 200 shares. That re-timestamping is why iceberg users effectively pay a queue-position penalty for hiding size.

Common Mistakes

1. Modifying an order in place and assuming priority is kept. On most venues, increasing the size of a resting order or changing its price loses time priority and starts a new timestamp. Decreasing size usually preserves priority. Every venue rulebook spells out which modifications preserve priority and which do not.

2. Confusing displayed priority with size priority. A 100-share displayed order beats a 10,000-share hidden order at the same price. Displayed liquidity is rewarded with the queue front, not size.

3. Assuming priority is portable across venues. Time priority is per-venue. A first-arrived bid at 50.00 on Nasdaq has no priority on NYSE Arca. Smart order routers must make explicit decisions about which venue's queue to join.

4. Ignoring the impact of fee tiers. Maker-rebate venues (BZX, EDGX) attract more passive flow, deepening queues. Inverted venues (BYX, EDGA) charge makers and pay takers, which thins their displayed queues and shortens expected wait times for joiners.

5. Treating priority as the only fill determinant. Even with perfect priority, your order may not fill if the spread is crossed only briefly. Effective queue position interacts with arrival rates, cancellation rates, and routing decisions made elsewhere in the market.

Frequently Asked Questions

Q: What are time priority and price priority in simple terms? When multiple orders sit at the same exchange, the matching engine fills the most aggressively priced order first. If two orders share the same price, the one that arrived first fills first. That is price-time priority.

Q: How do time priority and price priority affect investment decisions? For passive limit orders, they determine how long you wait in the queue before your order fills. For large investors, arriving earlier at a popular price level can mean the difference between filling before a move and being left on the queue when the price runs away.

Q: What is a real-world example of time priority and price priority? Orders A (500 shares, posted 9:30:01), B (200 shares, 9:31:44), and D (300 shares, 9:40:00) all rest at a $200.00 bid. A 1,000-share market sell arrives and fills 500 from A first, then 200 from B, then 100 from D. Priority is strict first-in-first-out within the price level.

Q: How can investors use time-price priority knowledge effectively? When using a GTC limit order that may sit for days, understand that hundreds of new orders may join the queue ahead of yours over time. If queue depth becomes very large, consider whether the order is still likely to fill before the thesis changes.

Q: How is price-time priority different from pro-rata allocation? Price-time priority fills the earliest order at the best price before any other. Pro-rata allocation, used on some futures and options markets, splits a fill proportionally among all orders at the same price. Most US cash equity exchanges use strict price-time, not pro-rata.

Sources

1. Nasdaq. "Equity 4, Order Ranking and Display Rules." https://listingcenter.nasdaq.com/rulebook/nasdaq/rules
2. NYSE. "Rule 7.36 Order Ranking, Display and Processing." https://nyseguide.srorules.com/rules
3. SEC. "Regulation NMS, Final Rule (Release No. 34-51808)." https://www.sec.gov/rules/final/34-51808.pdf
4. Cboe BZX. "Exchange Rulebook, Rule 11.12 Priority of Orders." https://cdn.cboe.com/resources/regulation/rule_book/BZX-Rules.pdf

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.