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Limit-on-close order

A limit-on-close order seeks the closing auction price within a specified bound. Separate price eligibility from allocation, deadlines and the handling of unfilled shares.

A limit-on-close order (LOC) requests participation in a closing auction at a price no worse than its limit. A buyer sets a maximum purchase price; a seller sets a minimum sale price. It lets a portfolio seek the closing benchmark while retaining a price bound, at the cost of possibly leaving the trade incomplete.
The limit is a permission to trade, not a promise of a fill. Acceptance, auction eligibility and allocation still matter. The IBKR Desktop explanation illustrates this order type; its interface does not support fractional-share LOC orders. Availability and routing depend on the broker, instrument and venue.

The price bound is only one test

Suppose an accepted LOC order buys 100 shares with a limit of USD 101 per share. Assume a fictional auction, whole shares and zero fees; the stated limit is unchanged by any venue controls.
  • If the auction clears at USD 102, no shares may fill: the price exceeds the authorised maximum. All 100 shares are unfilled.
  • In a different auction clearing at USD 100, the price condition passes. Suppose the allocation process awards this order 60 shares. Its purchase cost is USD 6,000, and 40 shares remain unfilled. It pays the auction price of 100, rather than its limit of 101.
  • At USD 101 exactly, the price condition also passes. Equality does not establish that all 100 shares receive an allocation.

These are conditional outcomes, not predictions of how the order changes the auction price. For a buy with effective limit LL and auction price PcP_c, a positive auction fill requires Pc≤LP_c\le L; for a sell it requires Pc≥LP_c\ge L. Neither inequality supplies a fill quantity.

Synthetic LOC buy of 100 shares capped at USD 101 per share. Auction A at USD 102 fails the price bound, so zero fill and 100 remain unfilled. Separate auction B at USD 100 passes; an assumed 60-share allocation costs USD 6,000 and leaves 40 unfilled.
Synthetic LOC buy of 100 shares capped at USD 101 per share. Auction A at USD 102 fails the price bound, so zero fill and 100 remain unfilled. Separate auction B at USD 100 passes; an assumed 60-share allocation costs USD 6,000 and leaves 40 unfilled.
Original synthetic alternatives for an accepted 100-share LOC buy with a USD 101 per share limit and zero fees. The auction prices and the 60-share allocation are stipulated, not solved. Unfilled quantity is not automatically a cancelled quantity.

An unfilled balance is an execution result. Whether it is cancelled, remains eligible for another closing-price process or continues under a different instruction is a separate lifecycle question.

LOC, MOC and an ordinary limit

An ordinary limit order imposes the same direction of price bound, but can be eligible in continuous trading under its session and lifetime instructions. Submitting one near the end of the day does not by itself make it auction-only.
A market-on-close order seeks the closing auction without a limit price. LOC adds price protection; it does not turn an uncertain auction price into a known one. MOC also remains subject to acceptance, available interest and venue controls.
SelfMadeQuant's Market, Limit and Conditional Orders develops the distinction between authorised price, completion and order state. The preview is public; the full lesson requires membership.

A venue clock, not just a closing time

Under Nasdaq Equity 4, checked 26 October 2026, regular-day LOC entry ends before 3:58 p.m. Eastern Time. From 3:50, cancellation/modification requires a legitimate-error request; none is permitted from 3:58. Entry from 3:55 requires a reference price and can face reference-price repricing or rejection.

Rule 4754 selects a cross price by maximum executable eligible volume, with tie-breaks and benchmark restrictions; it separately specifies allocation priority. Eligible price does not establish priority.

For Nasdaq-listed LOC orders entered through RASH/FIX, eligible unfilled shares can enter the Extended Trading Close unless the participant disables this. Executions remain at the official cross price within the limit. That process ends at 4:05 p.m. on regular days and cancels any residual. Other continuing time-in-force instructions can behave differently. These provisions require checking the actual route and order attributes.

Confirm the broker's own submission deadline and accepted instruction before treating a requested LOC as live. An acknowledgement and subsequent execution reports establish its state; a deadline printed in an exchange rulebook does not prove timely broker delivery.

Backtesting the decision honestly

Choose the order's side, quantity and limit using information available before its actual submission deadline. The final closing auction price can determine whether that already-submitted order satisfies its limit; it cannot be used retrospectively to choose an earlier order or its limit. Doing so introduces look-ahead bias.

A daily close below a buy limit establishes only a price comparison. It does not prove that the order was accepted, participated in that auction or received any particular quantity. A backtest needs a declared allocation assumption and a predeclared policy for unfilled shares, rather than silently recording the whole requested position.

Sources

  • Nasdaq Equity 4, Rules 4702(b)(12), 4754 and 4755: LOC acceptance and amendments, closing-price selection and priority, and eligible remainder handling; checked 26 October 2026. Times above describe a regular trading day.
  • Interactive Brokers — IBKR Desktop Limit on Close Order: broker order-type explanation and fractional-share restriction; checked 26 October 2026. Use the current venue rulebook and broker instructions for operational cutoffs.
  • Investor.gov — Types of Orders: direction of buy and sell limit-price protection; checked 26 October 2026.
  • Rishi K. Narang, Inside the Black Box: A Simple Guide to Quantitative and High-Frequency Trading, 2nd ed., Wiley, 2013, Chapter 7, “Other Order Types”, pp. 124–125: closing-auction order context and variation in exchange instructions. Numerical outcomes and the figure are original synthetic illustrations.
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