Order Book

Every time you buy or sell crypto on a trading platform, your order joins a long queue of other people trying to do the same thing. That queue is called the order book, and learning to read it can help you understand where prices come from and why they move.

What Does Order Book Mean?

An order book is a real-time record of every open buy order and sell order for a specific trading pair, such as BTC/USD or ETH/USDT. Buy orders are called bids, and sell orders are called asks (or offers). Each entry shows a price and the amount someone is willing to trade at that price. The exchange uses this list to match buyers with sellers automatically.

Think of it like a busy farmers market where every seller writes their asking price on a board, and every shopper writes the most they are willing to pay on another board. When a shopper's price meets a seller's price, a deal happens. The order book is simply those two boards, updated thousands of times per second.

How an Order Book Works

An order book has two sides that face each other. Bids are listed from the highest price down, because the buyer willing to pay the most gets served first. Asks are listed from the lowest price up, because the cheapest seller gets matched first. The point where these two sides meet is where trading happens.

Bids, Asks, and the Spread

The highest bid and the lowest ask are together called the top of the book. The gap between them is the spread. If the best bid for a coin is 99.90 and the best ask is 100.10, the spread is 0.20. A narrow spread usually means the market is active and competitive, while a wide spread often signals low liquidity and higher trading costs for you.

The Matching Engine

Behind every centralized exchange is software called a matching engine. It constantly scans the order book and pairs compatible orders based on price first and time second. If two people place a bid at the same price, the one who placed it earlier usually gets filled first.

Types of Orders You Will See

The orders you place decide how you interact with the order book. Here are the most common types:

• Limit order: You set the exact price you are willing to buy or sell at. Your order sits in the book until someone matches it, or until you cancel it.
• Market order: You buy or sell immediately at the best available price. Your order takes liquidity from the book instead of adding to it.
• Stop order: Your order activates only when the price reaches a trigger level you choose, which some traders use to limit losses.
• Post-only order: A limit order that is cancelled if it would match instantly, which guarantees you add liquidity to the book.

Makers and Takers

Traders who place limit orders that rest in the book are called makers, because they make liquidity available. Traders who fill those orders with market orders are called takers. Many exchanges charge makers a lower fee than takers to reward people for keeping the book full.

Why the Order Book Matters

The order book is more than a technical list. It tells you a lot about the health of a market and what a trade will really cost you.

Market Depth

Market depth describes how much volume sits at each price level. A deep book has large orders stacked close to the current price, so even a big trade barely moves it. A shallow book has only small orders, so a single large purchase can push the price up sharply. Many platforms show depth as a chart with green and red areas that rise on either side of the spread.

Slippage

When you place a large market order, it may eat through several price levels before it is fully filled. The difference between the price you expected and the average price you actually paid is called slippage. Checking the book before you trade helps you estimate how much slippage to expect.

Order Books on Centralized and Decentralized Exchanges

Most centralized exchanges use a traditional order book because it is fast and familiar to traders. The exchange runs the matching engine on its own servers and holds users' funds while they trade.

Decentralized exchanges take different approaches. Some run order books fully or partly on a blockchain, while many popular ones, such as Uniswap, replace the order book with an automated market maker. Instead of matching individual buyers and sellers, an automated market maker lets you trade against a pool of tokens with prices set by a formula. You can learn more in our guide to decentralized exchanges.

What to Watch Out For

An order book shows intentions, not guarantees. Orders can be cancelled at any moment, so a huge wall of bids today may vanish tomorrow. Some traders place large fake orders to create a false impression of demand or supply and then cancel them before they fill. This practice, known as spoofing, is a form of market manipulation.

Thin order books on small or new tokens are especially risky, because prices can swing wildly with very little volume. Before placing a large trade, look at the spread, check the depth on both sides, and consider using a limit order so you stay in control of the price you pay.

Useful Links

• What are the types of crypto exchanges?
• How do you start decentralized trading?
• What are OTC crypto markets?
• What is liquidity?