The National Stock Exchange (NSE) is set to revise the operational rules for its 15-minute pre-open auction session starting Monday, September 7, with revised rules for order placement, modification, cancellation and order matching. The overall schedule of 9 am to 9:15 am, however, will remain unchanged.
The updated framework reorganises the session into structured phases, splitting order placement into two distinct stages and restricting market order entries during the second window.
The primary objective of the revised framework is to align the opening price-discovery mechanism with the Closing Auction Session (CAS) rules introduced by SEBI in the equity cash market. The changes are also intended to improve operational consistency between cash and derivatives segments and facilitate smoother market execution.
While the overall pre-open session will continue to operate between 9:00 AM and 9:15 AM, the exchanges will change the way orders are handled during the first 12 minutes of the trading session.
Revised NSE pre-open session: What changes from September 7?
At present, investors can place, modify or cancel both market and limit orders between 9:00 AM and 9:08 AM. The order entry closes randomly between 9:07 AM and 9:08 AM, after which orders are matched and the opening price is determined.
From September 7, the process will be split into clearly defined stages. The order-entry period will be divided into two phases. Phase 1 will operate from 9:00 AM to 9:05 AM, during which order entry, modification and cancellation will be permitted for both limit and market orders.
In the Phase 2, from 9:05 AM to 9:10 AM, during which only limit orders can be placed, modified, or cancelled. Market orders will not be permitted during this period and will be rejected. The exchange may also apply a random closure during the final two minutes of this phase.
Following Phase 1, Phase 2 will run from 9:05 AM to 9:10 AM, allowing only limit orders to be placed, modified, or cancelled. Market orders are prohibited during this window and will be rejected. The exchange may also apply a random closure at any point during the final two minutes of this phase.
| Time | Current process | From September 7 |
| 9:00 AM–9:05 AM | Market and limit orders can be placed, modified or cancelled. | Market and limit orders can be placed, modified or cancelled. |
| 9:05 AM–9:08 AM | Market and limit orders can be placed, modified or cancelled. Order entry closes randomly between 9:07 AM and 9:08 AM. | Only limit orders can be placed, modified or cancelled. Market orders placed earlier cannot be modified or cancelled. |
| 9:08 AM–9:10 AM | Orders are matched and the opening price is determined. | Order entry will close randomly during this period. Order matching will begin immediately after order entry closes. |
| 9:10 AM–9:12 AM | Orders are matched and the opening price is determined. | Opening price is determined, orders are matched and trades are confirmed. |
| 9:12 AM–9:15 AM | Market transitions to regular trading. | Market transitions to regular trading. |
Following the order-matching window, a buffer period will run from 9:12 AM to 9:15 AM. This interval ensures a smooth operational transition from the pre-open auction into the continuous trading session.
Covered Securities Under Pre-Open
Operating via a call auction mechanism, the pre-open session applies across all securities in the equity market, including SME securities, partly paid-up securities, Infrastructure Investment Trusts (InvITs), and Real Estate Investment Trusts (REITs).
The pre-open process broadly splits into two core stages:
- Order Collection Period: A structured window allowing investors to enter, modify, or cancel orders.
- Order Matching Period: The phase during which orders are matched, and the opening price is determined.
Rationale behind NSE’s revision of the pre-open session framework
The exchange is updating the pre-open session framework to align its opening price-discovery process with the Closing Auction Session (CAS) mechanism introduced in the equity cash market on August 4.
The CAS system was established to enhance transparency and structural strength during closing price-discovery process. It applies to cash-market stocks with derivative contracts and uses a call auction model that aggregates buy and sell interest into a single liquidity pool to determine official closing levels.
According to SEBI, the unified auction framework improves order execution efficiency for large trades, facilitates fair settlement across derivative contracts and indices, and enables passive funds to transact at closing prices with lower tracking errors.
