What It Means
The Pre-Open Market Session is a 15-minute window (9:00–9:15 AM) before regular trading begins on NSE and BSE, that exists to discover a fair opening price and reduce the volatility of a cold market open.
How It Works
It runs in three phases: an 8-minute order collection window (9:00–9:08 AM) where you can place, modify, or cancel limit and market orders, and which closes at a random point between the 7th and 8th minute to prevent gaming; an order matching phase (roughly 9:08–9:12 AM) where no changes are allowed and the exchange computes a single equilibrium price that lets the maximum possible quantity trade; and a short buffer before regular trading opens at 9:15 AM.
Warning
Once the matching phase begins, you can't cancel or modify a pending order — get your order in during the collection window if you want it live for the pre-open price discovery.