Order Book

The order book is the beating heart of every exchange. On the NSE and BSE, it is the live, electronic register of every unexecuted buy and sell order for a stock, sorted by price and time.

How it works

The book has two sides. The bid side lists buyers and the prices they are willing to pay; the ask (or offer) side lists sellers and the prices they want. The highest bid and the lowest ask sit at the top, and the gap between them is the spread.

Indian exchanges run a price-time priority system. The best price gets filled first, and among orders at the same price, whoever placed theirs earlier goes ahead. When a buyer's bid meets a seller's ask, a trade prints and both orders leave the book.

Most retail traders see only the market depth window, which shows the top five bid and ask price levels along with the quantity sitting at each. Brokers like Zerodha and Groww display this on their terminals.

In India

NSE and BSE operate fully order-driven, anonymous order books. You do not know who is on the other side, and the matching engine is automated. SEBI rules around tick sizes, circuit limits, and a single price during the pre-open call auction all shape how the book behaves through the day.

Liquid large-caps like Reliance or HDFC Bank have deep books with thin spreads, while small-caps may show wide gaps and lumpy quantities, a warning sign of poor liquidity.

Why it matters

Reading the book helps you judge how much you can buy or sell without moving the price. A thin order book means even a modest order can push the price sharply, leading to bad fills.

Common mistakes

Beginners chase what looks like a big buy wall, assuming it guarantees support. Those quantities can be cancelled in milliseconds, and high-frequency players routinely place and pull orders. Treat the book as a snapshot of intent, not a promise.