Order book: the live ledger of buyers and sellers

How an Order Book Works

An order book is the real-time list of every outstanding buy and sell order for a given trading pair, organized by price level. The bid side aggregates buyers willing to pay each price; the ask side aggregates sellers willing to part with their tokens at each price. The exchange's matching engine continuously pairs the best bid with the best ask, executing trades as soon as the two prices cross.

The gap between the highest bid and the lowest ask is the bid-ask spread. Tight spreads and deep books at each level signal a liquid market where large trades can execute with minimal price impact. Thin books mean even small orders can move the price meaningfully and slippage will be high.

Order books also reveal market structure. Stacked walls of orders at round numbers often act as psychological support or resistance; sudden vanishing of resting orders can hint at incoming volatility. Decentralized exchanges based on automated market makers work very differently—they replace the order book with a mathematical pricing curve—but most centralized venues, derivatives platforms, and institutional trading still rely on the classic order book design.