Guide · Market structure · Updated 2026-09-23

An auction order takes a price it cannot see, and since 7 September the pre-open fills market orders first

Most of what a trader knows about orders was learned in continuous trading, where an order is tested against the book the instant it arrives and either trades at prices already on the screen or waits in a queue it can leave at will, as the guide to queue position sets out. Twice a day none of that holds. An order sent at 9:02 or at 3:22 carries the same fields as one sent at 10:30 and is a different instrument: it names no price it will get, it cannot always be withdrawn, and when its side of the book is oversubscribed, a sequence its owner never sees decides whether it is filled.

This guide follows three retail orders through one illustrative auction book, puts a market order into a thin book, and measures how far the opening price has actually landed from the last price an overnight order could see. How the clearing price is computed is worked in the guide to the pre-open call auction, and what the closing auction does to the close in the guide to the closing price; neither is repeated here.

The ticket is the same, the contract is not

In continuous trading a buy limit order at 503.00 against a best offer of 501.00 trades at once, at 501.00. In an auction window the same ticket is collected and held. Nothing matches until entry closes; then the exchange computes one equilibrium price, the price at which the most shares can change hands, and matches orders at that price and no other (SEBI circular of 16 January 2026, paragraph 4.6, for the close; paragraph 17.1 of Chapter 1 of SEBI's master circular for stock exchanges, as that circular amended it, for the open).

Entering an auction therefore commits an order to three things continuous trading never asks of it. A price not yet known: the indicative price on the screen is the answer the book would give if entry closed that instant, recomputed as orders arrive, so a market order entered while it reads 500.50 is a promise to accept whatever it reads when entry shuts. A lock: after 9:05 in the morning and 3:25 in the afternoon a market order cannot be changed or cancelled, and after the random close nothing can. And a rationing rule: when more shares are bid than offered at the clearing price, or the reverse, part of the heavy side goes unfilled, and a sequence written into the circulars decides which part.

The refused order types follow from the batch. A stop loss needs a trade to trigger on, and nothing trades before matching; immediate or cancel needs an immediate match, and none exists; a disclosed quantity order hides size from a book built to aggregate it. All three are rejected in both auctions (NSE/CMTR/74969, note 1(a); NSE/CMTR/74466, note (a)(i)), and there is no market on close or limit on close order (question 21 of the exchange's closing auction FAQ, version 2.0).

What each window accepts, and the minute it stops listening

The lock is set by order type, not by the clock alone. A market order entered at 9:04 is frozen at 9:05 until it is matched or moved into continuous trading at 9:15, while a limit order entered in the same second can be repriced or withdrawn until the random close. Before 7 September there was no separate lock: both kinds could be changed until one random close between 9:07 and 9:08 (NSE/CMTR/73927, section 1.1). The close is random inside its band, so a change sent at 9:08:40 or 3:28:40 may arrive after entry has shut and be rejected (question 38 of the same FAQ), and once entry shuts nothing can be modified or cancelled before continuous trading (pre-open FAQ, question 9).

Nothing outside the window reaches back in. Trades struck in either auction cannot be cancelled on request (NSE/CMTR/74969, note 2; NSE/CMTR/74466, note (d)). The exchange's cancel on logout and kill switch facilities do not touch pre-open orders (pre-open FAQ, question 13; NSE/CMTR/73927, section 3.5), so a broker's emergency withdrawal leaves them in the book. Market price protection, the exchange's usual cap on how far a market order can trade from the last price, does not apply in either auction (NSE/CMTR/73927, section 3.6; NSE/CMTR/74466, section K). A market order's only limit is the price band: the share's normal band in the pre-open (pre-open FAQ, question 3) and 3 per cent either side of the reference price in the closing auction.

The afternoon has one event the morning lacks. At 3:15, when continuous trading ends for shares with derivatives, resting limit orders inside the new band are carried into the auction with their original time stamps, ahead of limit orders entered during it, while stop loss orders, disclosed quantity orders and anything priced outside the band are cancelled (SEBI circular, paragraph 4.8; NSE/CMTR/74466, section E; FAQ question 15). A protective stop on such a share does not survive into the auction, and the exchange takes no other until continuous trading the next morning. Any order that reaches the exchange between 3:15 and 3:20 is rejected, whatever sent it, including a trigger held at the broker, the arrangement examined in the guide to broker-side triggers.

One price for every matched share

Take a pre-open book in a share whose previous close is 500.00. It is illustrative. Three retail orders sit in it: R1, a market buy for 50 shares entered at 9:00:45; R2, a limit buy for 50 at 503.00 entered at 9:06:50, after the market order window had shut; and R3, a limit buy for 50 at 499.50 entered at 9:01:00. Around them are a fund's market buy of 900, six other limit buys listed in the table further down, a market sell of 300 entered at 9:01:40, and limit sells of 500 at 499.50 (9:00:10), 700 at 500.50 (9:00:20), 900 at 501.00 (9:03:20), 500 at 501.50 (9:06:30), 800 at 502.50 (9:02:40), 1,000 at 503.50 (9:01:20) and 1,500 at 505.00 (9:06:40).

Run the published rules on the book as it stood at the random close and one price clears more shares than any other: at 501.50, 3,200 shares are bid and 2,900 offered, so 2,900 trade, and no other price lets more than 2,400 trade. That is the day's opening price, and the price of every share that trades in the auction.

R1 was a promise to accept whatever cleared. When it was entered the indicative price on the book then standing was 500.50; at 9:05, when R1 could no longer be changed, it read 501.00; limit orders arriving after that took it to 501.50, which is what R1 paid, 1.00 more than the screen showed when it was sent and 0.30 per cent above the previous close. Later orders could as easily have moved the price the other way. The market order had no say in either direction.

R2 was a promise to pay no more than 503.00. It paid 501.50, like every other buyer, ₹75 less than its limit on 50 shares. That is why a limit order is the natural way to take part in an auction: it states the worst price its owner will accept and then, if it trades, takes the clearing price, which can only be the same or better.

R3, at 499.50, is below the clearing price and does not trade. In the pre-open it moves into the continuous book at 9:15 at 499.50 with its 9:01:00 time stamp, ahead of any order at that price entered later (NSE/CMTR/73927, section 1.1 F). The same order in the closing auction would be cancelled when matching ended (NSE/CMTR/74466, section G.4).

At 501.50 buyers want 300 more shares than sellers offer, so some buyer goes short. Which buyer that is depends on a rule the regulator changed on 7 September.

On 7 September the queue for the heavy side was turned round

When one side is heavier at the clearing price, the exchange fills the lighter side completely and rations the heavier one, in an order written into the execution sequence. Until Friday 4 September the pre-open matched limit orders first: eligible limit orders against each other, then the remaining limit orders against market orders, then market orders against each other, as the exchange's consolidated circular of 28 April 2026, NSE/CMTR/73927, states in section 1.1. From Monday 7 September, SEBI's circular of 16 January 2026 (paragraph 5.1, rewriting paragraph 17.1.9 of Chapter 1 of the master circular) gives market orders priority: eligible market orders are matched with each other in time priority, the remaining market orders with limit orders in price and time priority, and only then the remaining limit orders with each other (NSE/CMTR/74969 of 1 July 2026, section B). That is the sequence the closing auction has used since it opened on 3 August (SEBI circular, paragraph 4.7; NSE/CMTR/74466, section G). For 25 sessions, 3 August to 4 September 2026, the exchange ran its two daily auctions on opposite priority rules.

All 2,900 shares trade at 501.50 under both sequences; only their owners change. Under the old sequence the limit buyers were served first, highest price first, and the shortfall landed on the market orders: the fund received 650 of its 900 and R1 received nothing, its 50 shares entering continuous trading at 9:15 as a limit order at 501.50. Under the current sequence the market orders are served first and the shortfall lands on the limit order priced exactly at the clearing price: B3, which bid 501.50 for 1,200 shares, receives 900.

Three consequences follow. A market order on the heavy side is now close to certain of a fill; it goes short only when the market orders on its side exceed everything the other side offers at the clearing price. A limit order priced exactly at the clearing price is the residual claimant, last in price priority among the orders able to trade and so the most likely to be part filled. And a participant now chooses between priority and the right to change their mind. A limit buy at the top of the band carries the same worst price as a market order, since no auction clears outside its band, and stays changeable until the random close, but it queues behind every market order on its side. A market order buys first place and pays for it with the lock at 9:05 or 3:25. The change of 7 September gave the market order the front of the pre-open queue and took away its last two to three minutes of flexibility at the same time.

Where the part that does not fill goes

A partial fill is ordinary in an auction, and the two sessions treat the remainder in opposite ways.

In the pre-open one detail is unsettled in the exchange's own documents. The consolidated circular, which the July 2026 circular leaves unchanged on this point, says unmatched orders move to continuous trading retaining their original time stamps, market orders at the clearing price; the FAQ attached to the July circular says market orders move with a modified time stamp, and the FAQ itself says the circulars prevail where the two differ. Either way the result is a resting limit order at the auction price, a different position from the one a market order was meant to produce: from 9:15 it waits for sellers at that price while the market moves on.

After the closing auction, every order the matching did not execute, and the unexecuted part of every order it executed in part, is cancelled by the exchange (NSE/CMTR/74466, sections B(c) and G.4). Nothing carries into the post close session or the next day. A participant who still wants the shares at 3:35 has one venue left that day: the post close session from 3:50 to 4:00 pm, which takes only market orders and fills them only at the closing price and only against someone on the other side (SEBI circular, paragraph 4.2.4; NSE/CMTR/73927, section 1.8). Shares without derivatives are not in the closing auction; they trade continuously to 3:30 and close on the average price of their last half hour.

An imbalance moves the price against the side that caused it

The heavy side in the first book was rationed at a price the whole book agreed on. A large enough imbalance moves the price itself, and every share on the side that caused it pays the new price.

Take a closing auction in a share whose reference price is 500.00, so the band runs from 485.00 to 515.00; it is illustrative. The bids are the same in two versions of the book: 2,000 shares at 500.00 and at each of the four prices 0.50 apart below it. In the deep version 2,000 shares are offered at 500.00 and at every 0.50 up to 515.00. In the thin version 250 shares are offered at 500.00 and at every rupee up to 515.00, 4,000 in all. Both books clear at 500.00 before a market buy is added.

In the deep book the order barely registers: of the sizes tried, 2,500 shares is the first to move the price, by 0.10 per cent. In the thin book every size moves it, and 4,000 shares take it to the band edge. The mechanism is the single price. A market order that walks up four levels in continuous trading pays each level's price in turn; in the auction the same four levels set one price, the last, and every share pays it. The order's last share sets the price of its first.

In the thin book a 1,000 share market buy pays 503.00 on every share: ₹3,000 above the price the book showed before it arrived, and ₹1,500 more than walking the same offers, which averages 501.50. The factor of two belongs to this evenly spaced ladder, not to auctions in general; the direction is general.

The first thin row is the surprise. A market buy of 250 shares, a modest retail order, moves the clearing price from 500.00 to 501.00 without adding a share to the matched volume. Without it, 250 shares trade at 500.00 and 1,750 shares of bids are left over; with it, 250 shares can trade at either price, and the rule, finding the volume tied, picks the price with the smaller unmatched quantity. The market order has displaced the limit buyers at 500.00 and raised its own price by a rupee. That row assumes, as the exchanges' published worked examples do, that only prices at which orders rest are candidates; were every tick a candidate it would move one tick, to 500.05, and no other figure here would change.

At 6,000 shares the order exceeds the 4,000 offered inside the band: the price stops at 515.00, 4,000 shares trade and 2,000 are cancelled when matching ends. In the pre-open the remainder would instead rest in the continuous book at the clearing price, with the share's normal band as the ceiling.

The exchange disseminates the imbalance while entry is open, at the indicative price and for market orders alone (SEBI circular, paragraphs 4.12 and 5.1; NSE/CMTR/74969, section C). A seller who sees a large unmatched market buy has a reason to offer, and when sellers respond the price comes back towards the reference, as the worked example in the closing price guide shows. The table assumes nobody responds. An order that relies on the response is relying on other people's reading of a number it created.

Orders queued overnight meet the auction with yesterday's information

An after-market order is a broker facility, not an exchange order type. Many Indian brokers hold equity after-market orders overnight and release them to the exchange when the pre-open opens at 9:00, where they are collected like any other order; the windows are set out in the guide to trading hours. The auction's rules apply to them in full, and three bite harder on an order nobody is watching. A market after-market order released at 9:00 is locked at 9:05, so an owner who is not at the screen in those five minutes cannot withdraw it once the broker has released it; since 7 September it is also first in line. A stop loss cannot enter the pre-open at all, since the exchange refuses stop loss orders until continuous trading. And time priority starts when the exchange accepts an order, not when its owner typed it: an order placed at 8 pm and released at 9:00 ranks with everything else released at 9:00.

Since 7 September the pre-open has also taken in gold and silver exchange traded funds, whose underlying metals trade abroad overnight while the funds trade only in Indian hours; other exchange traded funds still open on their first continuous trade (SEBI circular of 15 June 2026, paragraph 7, in force from 7 September under its extension of 28 August 2026; NSE/CMTR/76101).

The price an overnight market order accepts is the opening price; the last price it could see was the previous close. How far apart those two have been is measurable, and it depends heavily on how much a share trades.

Across 2,274,968 stock sessions from January 2022 to 18 September 2026, the median distance was 0.40 per cent in shares that usually trade 100 crore or more a day and 0.95 per cent in shares that trade under 0.1 crore. In that thinnest band 26.9 per cent of sessions opened more than 2 per cent away, and one in a hundred more than 8.19 per cent away: the range of prices an overnight market order in such a share has agreed to without seeing any of them.

The last two columns record how often the opening price, the auction's price whenever the auction found one, was the day's extreme. In the thinnest band it was the high in 27.5 per cent of sessions and the low in 12.5 per cent; in the most traded, 6.2 and 3.6 per cent. On those days every buyer matched at the open paid the highest price the share printed all day, or every seller received the lowest. The columns are not symmetric in any band; the measurement records that without explaining it, and it is not a forecast.

When the auction is the right place for an order, and when it is not

The auction is the right place for an order when the price it produces is the price wanted: the official close in a share with derivatives, which the index, fund valuations and derivatives settlement use that evening; or simply the clearing price, for an order small against the auction's volume in a liquid share, which takes the price others set and often, like R2, a better one than it asked for. A continuous order is simpler whenever control matters more than the price being official: when the trade needs a stop, a cancel or immediacy; when the share is thin enough that the order would itself be the imbalance; and when its owner cannot watch the window in which it locks.

What an auction order actually buys

An auction order buys a share of one price, a place in a queue whose order is fixed by circular, and a set of moments after which the order is no longer its owner's to change. None of that appears on the order ticket, which looks the same at 9:02 as at 10:30. Knowing which rulebook an order is about to enter, and what it gives up by entering, is execution judgement of the kind the curriculum teaches as mechanism rather than as habit.