Order Priority and Price Formation in Opening Auctions

Summary

The document considers how an exchange matches a resting buy and sell order at the market open when their prices overlap and their arrival times appear simultaneous. It uses a simple example with a seller willing to transact below the buyer’s limit, and asks which execution price is natural when time priority cannot distinguish the orders.

The answers emphasize that matching rules depend on the exchange and may differ between a listing venue and other markets. One answer points to the Nasdaq opening auction as an example, while another explains that orders with identical timestamps may be sequenced using the exchange-assigned order identifier. These points show why traders should consult the relevant venue’s auction and priority rules rather than infer a universal price from the two limit prices alone. The document does not specify the Nasdaq auction’s complete pricing algorithm or establish that order-number priority governs every exchange.

Key ideas

• Exchange rules determine how opening-auction orders are matched and priced.
• The two limit prices alone do not determine a universal execution price when arrival priority is tied.
• An exchange may use a unique order identifier to sequence orders with identical timestamps.
• Auction rules can vary across venues, so venue-specific documentation matters.

Tags

# How are piled-up orders matched?


# How are piled-up orders matched?












Suppose that the market is closed, but orders are queued up to be matched as soon as the market opens. Suppose for simplicity that only two orders are queued:

- A sell order to sell 1 unit at \$1.00

- A buy order to buy 1 unit at \$1.10

If order 1 came in before order 2, the orders are matched at \$1.00, and if order 2 came in before order 1, the orders are matched at \$1.10. But in this case both orders come in at exactly the same time, and hence I don't know what a natural price would be for this transaction?

## Answer by Bikenfly (score 4, accepted)

https://quant.stackexchange.com/a/66023

Each exchange is a bit different, especially the listing exchange vs others, so no one answer. This should add a bit of color on the Nasdaq opening auction -

## Answer by Sergei Rodionov (score 1)

https://quant.stackexchange.com/a/66039

If timestamps are the same, the orders will be processed based on order number, a unique identifier assigned by the exchange at the time each order is added to the queue.

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.