Call auctions and uncrossing
Your limit buy at a high price filled at a much lower one, in a single trade, at the same instant as everyone else’s. Or your order sat at exactly the price everything traded at and received nothing.
Both are ordinary outcomes of a call auction, which is the other way to run a market. Continuous matching applies each instruction as it arrives; an auction accumulates instructions without matching any of them, then computes one price and trades everything that can trade at it. The rules are different enough that expectations carried over from continuous trading are usually wrong.
What a call auction is
Two phases.
The call. For some interval, orders can be submitted, amended and cancelled, and none of them match. The book accumulates and is allowed to be crossed — bids above asks — which is impossible in continuous trading because the engine would have matched them.
The uncross. At the end of the interval the venue computes a single clearing price and executes every order that can trade at it. All resulting trades happen at that one price, simultaneously, as one event.
The second phase is where the surprises come from, and there are two: every fill is at the same price regardless of the limit prices submitted, and orders resting exactly at the clearing price may or may not trade.
How the clearing price is chosen
The standard objective is maximum executable quantity: the price at which the most units can change hands.
Constructing it is bookkeeping. For each candidate price, sum all buy interest willing to pay that price or more, sum all sell interest willing to accept that price or less, and take the smaller of the two — that is what could trade there. Do it for every candidate and pick the largest.
Ties are common, and venues break them with a published sequence of further criteria. Typical ones, in some order: minimum imbalance — of the tied prices, choose the one leaving the least quantity unexecuted; market pressure — choose the side with the surplus and favour it; and reference price proximity — choose the candidate closest to some prior price, often the last continuous trade.
Which criteria apply and in which order is a venue decision, and on a tie it genuinely changes the price everybody trades at.
What happens at the clearing price itself
Orders strictly better than the clearing price all execute in full, because they were willing to accept something better than they asked for. That is the mechanism paying them the difference: a buy limit above the clearing price still pays the clearing price.
Orders at the clearing price are the marginal ones, and only some of them can trade — by construction, since the two sides do not balance exactly at that price. They are allocated by the venue’s rule, usually price-time or pro-rata applied within the level.
So a limit order at exactly the auction price can fill in full, fill partially, or not fill at all, depending on where it sat in the allocation. There is no version of the rule where every order at the clearing price trades.
Unexecuted orders are then either carried into continuous trading, cancelled, or expired according to their time in force — again a venue decision, and one that determines whether an unfilled auction order becomes a resting order you now own.
Indicative prices during the call
Many venues publish, during the call phase, what the clearing price and quantity would be if the auction uncrossed at that instant. This is often called an indicative or theoretical price.
It is a real calculation on a real book and it is not a prediction of the outcome. Every order in the auction can be cancelled or amended before the uncross, so the indicative figure describes a state that nobody is committed to. A large order arriving in the final moments changes it, and so does a large order leaving.
Some venues therefore randomise the exact end of the call within a short window, so that the closing instant cannot be targeted. Whether that is done, and over what window, varies.
The mechanism
THE MECHANISM — a call auction
· Order submitted during the call
→ accumulates. NOTHING matches, and
the book may become crossed.
· Uncross
→ one price computed, all eligible
orders trade at it simultaneously.
· Limit better than the clearing price
→ fills in full, AT the clearing
price. You receive the difference.
· Limit exactly at the clearing price
→ marginal. NO GUARANTEE of a fill;
allocated by the venue's rule
within the level.
· Indicative price during the call
→ a calculation on an uncommitted
book. Not a forecast of the
uncross.
· Unexecuted order after the uncross
→ carried, cancelled or expired.
You may now hold a resting order
you did not plan for.
· Tie-break criteria, call length, closing
randomisation and which events use an
auction
→ VENUE-SPECIFIC. Tie-breaks change
the price everyone trades at.Worked example
Illustrative figures, synthetic throughout. Suppose an auction closes with this collected interest.
Buy orders: 3.0 units at 40,020, 2.0 at 40,010, 5.0 at 40,000. Sell orders: 1.0 unit at 39,990, 4.0 at 40,000, 2.0 at 40,010.
Cumulative buy interest at each candidate price, counting every buyer willing to pay at least that much: 40,020 → 3.0; 40,010 → 5.0; 40,000 → 10.0.
Cumulative sell interest, counting every seller willing to accept at most that much: 39,990 → 1.0; 40,000 → 5.0; 40,010 → 7.0.
Executable quantity is the smaller of the two at each price: at 39,990, 1.0. At 40,000, 5.0. At 40,010, 5.0. At 40,020, 3.0.
The maximum is 5.0, and it occurs at two prices — a tie between 40,000 and 40,010. Apply a minimum imbalance rule: at 40,000 the surplus is 10.0 buy against 5.0 sell, leaving 5.0 unexecuted; at 40,010 it is 5.0 buy against 7.0 sell, leaving 2.0. The smaller imbalance is at 40,010, so that is the clearing price.
Now the fills. The buyer at 40,020 pays 40,010 — better than their own limit, in full, for 3.0. The buyer at 40,010 fills 2.0, in full, at their limit. Buyers at 40,000 get nothing; the price cleared above them.
On the sell side, 39,990 fills 1.0 at 40,010, well above its limit. The 4.0 at 40,000 fills at 40,010. That accounts for the whole 5.0 traded — so the 2.0 resting at exactly 40,010 receives nothing, despite being at the clearing price, because the quantity was already satisfied by better-priced sellers.
Same book, one event, and three different experiences: filled better than asked, filled as asked, and sitting at the trade price with no fill.
Why a venue would use one
Auctions solve a problem continuous matching cannot. When a market is reopening after a halt, or opening after an interruption, the accumulated interest is likely to be one-sided and the book unrepresentative. Matching that continuously means the first arriving order transacts against whatever thin quantity is there, printing a price that the subsequent flow immediately contradicts.
An auction removes the advantage of being first, because everything inside the call is treated as simultaneous. That is the same reasoning behind batched matching — arrival order stops deciding outcomes — applied at a single point rather than continuously.
It trades that away for two things: a delay between submitting and knowing, and a concentration of the entire event into one price that a late order can move.
The failure mode
The characteristic trap is treating an auction limit price as a description of what you will pay. It is only a boundary. You may pay considerably better, and the amount is determined by everyone else’s orders rather than yours.
The sharper failure is the marginal one. An order at the clearing price is the one most likely to be partially filled or skipped, and it is precisely the order someone places when they are trying to transact “at the auction price” — an intention the mechanism does not support, because the price is not known when the order must be submitted.
And the indicative figure is not a commitment by anyone. A decision made on it, in the last moments of a call, is a decision made about a book that can still be dismantled before it clears.