Order Book

What Is the Order Book?

The order book is a real-time, organized list of all pending buy and sell orders for a security. It provides a transparent view of the supply and demand landscape at every price level, showing exactly how many shares or contracts market participants are willing to buy or sell and at what prices. The order book is the core mechanism through which securities are traded on modern electronic exchanges.

At the center of the order book are the best bid (highest price someone is willing to pay) and best ask (lowest price someone is willing to accept). The difference between these is the bid-ask spread. Below the best bid, additional buy orders queue at progressively lower prices. Above the best ask, sell orders queue at progressively higher prices.

How the Order Book Works

When a market order arrives, it is matched against the best available orders on the opposite side of the book. A market buy order fills against the best ask. If the order size exceeds the quantity available at the best ask, it "walks the book," filling at successively higher price levels until the entire order is complete.

Limit orders rest in the book at their specified prices, waiting to be matched. The order book operates on price-time priority: orders at the best price are filled first, and among orders at the same price, earlier orders have priority.

The depth of the order book at each price level reveals the strength of support or resistance. A large cluster of buy orders at a price level suggests strong demand (support), while a large cluster of sell orders suggests strong supply (resistance).

Reading Order Book for Trading Signals

Order imbalance between the bid and ask sides can signal short-term directional pressure. If the bid side shows significantly more quantity than the ask side near the current price, short-term upward pressure is likely as buying demand outweighs selling supply.

Large resting orders at specific price levels can act as walls that temporarily prevent price from moving through that level. Traders watch for these walls to be absorbed (a bullish sign if a large ask is consumed) or pulled (a potentially bearish sign if large bids are cancelled).

However, the visible order book represents only a fraction of true supply and demand. Hidden orders, iceberg orders, and orders on dark pools are not visible in the order book, which limits its predictive value.