Order Book

The order book (also called market depth or Level 2 data) is a real-time electronic list of all outstanding buy and sell orders for a security, organised by price level. It provides a transparent view of supply and demand at every price point, revealing the Liquidity structure of the market.

On Indian exchanges, the order book shows buy orders (bids) on one side and sell orders (asks/offers) on the other. The best bid (highest buy price) and best ask (lowest sell price) define the "top of book." The difference between them is the bid-ask spread — a key measure of liquidity. For Reliance on the NSE, the spread might be just ₹0.05 (extremely liquid), while a small-cap might have a ₹2–5 spread.

Most Indian broker platforms display 5 levels of market depth — the best 5 bid and ask prices with the quantity available at each level. NSE also offers a 20-depth feed for subscribers. For example, you might see: Bid side — ₹2,500.00 (5,000 shares), ₹2,499.95 (3,200 shares), ₹2,499.90 (8,100 shares); Ask side — ₹2,500.05 (4,500 shares), ₹2,500.10 (2,800 shares), ₹2,500.15 (6,000 shares).

The order book is the mechanism through which Limit Orders and Market Orders are matched. The NSE's matching engine follows price-time priority: the order offering the best price is matched first, and among orders at the same price, the one placed earliest gets priority. This matching engine processes millions of orders per day with latency in microseconds.

For traders, reading the order book reveals important clues. A thick bid wall (large quantity at a specific bid price) suggests strong buying support at that level. A large ask wall signals resistance. However, sophisticated traders practice "spoofing" — placing and quickly cancelling large orders to create false impressions of supply/demand. SEBI has regulations against such manipulative practices, though enforcement is challenging.