On Monday the market opens on a different clock: the pre-open session is rebuilt from 7 September 2026

What is different on Monday morning

Until Friday the pre-open collected orders in one block and then matched them. From Monday it collects them in two blocks with different rules, and matching starts later. The table is the whole change; everything after it is what the two circulars say about the rules inside each stage.

The session is the same length. The order window inside it is now split in two, and the half you can put a market order into is the shorter one.

The 5 minutes that changed the most

The single most practical consequence is about market orders. From 9:05 am the exchange stops accepting them, and it also stops accepting changes and cancellations to the ones already sitting in the book. A market order placed at 9:00 am is therefore committed for the rest of the auction.

• Limit orders and market orders both count towards the opening price, in the stage where each is accepted.
• Quantity cannot be hidden. An order must be disclosed in full, so iceberg orders are not accepted.
• The exchange's special-term order types are not accepted at all: the kind that rests until a trigger price is reached, the immediate-or-cancel order, and the disclosed-quantity order.
• Algorithmic market orders are accepted only during the first 5 minutes — the one stage in which market orders are allowed.
• A market order placed after that stage is rejected by the exchange, with the message “Market orders are currently not allowed.” on the trading terminal.
• A trade executed in the pre-open cannot be cancelled. The request is rejected by the system.

How the opening price gets chosen

Nothing executes while orders are being collected. At the end the system looks for the single price at which the largest quantity can be matched, and every trade in the auction happens at that one price. That mechanism is not new — what the circular restates is the order in which the matching engine works through the book.

1. Eligible market orders are matched against each other first, in time order, at the final opening price.
2. Whatever is left of them is matched against limit orders, in price-then-time order.
3. The remaining limit orders are matched among themselves, in price-then-time order.

The order entry period does not end on the clock. It shuts at a system-driven moment somewhere between 9:08 am and 9:10 am, and matching begins as soon as it does. The exact instant is not knowable in advance, by design.

A random close means the last minute of the order window is not a minute anyone can plan around. That is the point of it.

What the screen shows while it runs

There is no last traded price during a call auction, because nothing has traded. The exchange publishes a different information set instead, and the circulars list it item by item.

• The indicative opening price of the stock, and the quantity tradable at it.
• The indicative cumulative buy and sell quantity.
• The indicative imbalance at the equilibrium price.
• The indicative imbalance counting market orders alone.
• The indicative opening value of the indices, NIFTY 50 included.

The last item is the one worth noticing: an indicative opening value for the indices, NIFTY 50 included, is published while the auction is still collecting orders. It is indicative — it is what the index would open at if the auction closed on the current book, not a value anything traded at.

Why the pre-open moved at all

This is the second half of a circular whose first half already landed. The same SEBI document introduced the closing auction in the cash segment, implemented on 3 August 2026, and rewrote the pre-open to match it — the same stage structure, the same random close, the same two-block order window, at the other end of the day.

From Monday both ends of the trading day are call auctions built the same way. That symmetry is the reason the change exists.

Limits, honestly stated

• This post describes what the circulars say. It does not describe how any particular broker exposes the pre-open in its order window, or which of these order types a broker chooses to offer at all.
• The times are the exchange's published schedule for an ordinary trading day. A special session, or a day that reopens after a market-wide halt, has its own timings notified separately.
• Later circulars can amend any of this. Each source below carries the date a human last read it.
• This is a description of market structure, not advice, and nothing here says what anyone ought to do about it.