Order Book
Ranked list of open buy and sell orders for a market.
An order book is a live list of open instructions to buy and sell an asset, arranged by price and often by submission time. Buy orders are called bids, while sell orders are called asks or offers. When compatible prices meet, the trading venue's matching rules determine which orders execute and in what sequence.
The highest bid and lowest ask form the top of the book. Their difference is the bid-ask spread. Additional price levels show market depth, meaning how much quantity is available as price moves away from the midpoint. A narrow spread and deep book often support cheaper execution, but displayed liquidity can disappear quickly and does not guarantee a specific fill.
Order books matter because they reveal current trading interest and support price discovery. A trader can place a limit order at a chosen price or use a market order to trade against available orders immediately. Large orders may consume several price levels, producing an average execution price worse than the first quote. This difference is one form of slippage.
Centralized exchanges maintain order books in private systems and update user balances internally. Some decentralized exchanges store orders and matching logic on-chain, while others distribute signed orders off-chain and settle trades through contracts. On-chain books offer public visibility but may face higher costs and latency. Off-chain matching is faster, yet adds operator availability and fairness considerations.
Visible depth can be misleading. Traders may place and cancel large orders to create a false impression of demand, a practice known as spoofing in regulated markets. Bots can react faster than human users, and wash trading can inflate activity. Some venues also support hidden or iceberg orders, so the public book may show only part of available interest. Historical volume does not prove current liquidity.
Before trading, examine spread, cumulative depth, recent executions, fees, minimum size, and order priority rules. Use limit prices for control and divide large orders when appropriate, while recognizing that repeated orders can reveal intent. Compare expected average fill rather than the last traded price. An order book is a useful market map, but execution quality depends on real liquidity, venue rules, and changing conditions.
Frequently asked questions
- The spread is the difference between the highest bid and lowest ask. A wider spread raises the immediate cost of crossing the market and often signals weaker liquidity or greater uncertainty. A limit order can avoid paying the full spread, but may not execute. Compare the expected fill price, fees, and slippage rather than looking only at the displayed spread.
- Market depth is the amount available at multiple bid and ask price levels. A deep book can absorb a larger trade with less price movement, while a thin book may show a good top price but little quantity behind it. Traders should examine cumulative depth and estimated average fill because visible orders can be canceled before execution or partly hidden.
- Some decentralized exchanges use on-chain order books, while others keep signed orders off-chain and settle matched trades through smart contracts. Many DEXes instead use automated market maker pools. Each design changes fees, latency, custody, transparency, and front-running risk. Users should confirm where orders are stored, who matches them, and what permissions are granted during settlement.