An order book is a real-time list of all open buy and sell orders for a trading pair on an exchange. The buy side (bids) shows the prices at which people are willing to buy and how much they want to buy at each price. The sell side (asks) shows the prices at which people are willing to sell and their quantities. The gap between the highest bid and lowest ask is the spread.
The spread itself tells you something about liquidity and volatility. Tight spreads (a few cents on major pairs like BTC/USDT) indicate deep liquidity and competitive market making. Wide spreads suggest thinner markets where fewer participants are actively quoting. Spreads tend to widen during high volatility because market makers increase their compensation for taking on more risk.
Depth refers to how many orders are stacked at various price levels away from the current price. A deep order book has substantial volume at each level, meaning large orders can be filled without moving the price much. A thin order book means even modest orders can cause significant price impact. You can visualize depth using a depth chart, which plots cumulative bid and ask volume against price.
Large orders sitting on one side of the book can indicate support or resistance levels. A wall of buy orders at a specific price suggests strong demand at that level. A wall of sell orders indicates supply that needs to be absorbed before the price can move higher. But these walls can be deceptive. They are sometimes placed as spoofing tactics and pulled before they are actually filled.
The order book is constantly changing. Orders are placed, cancelled, and filled continuously. Watching the flow of these changes, known as order flow analysis, can reveal intentions that the static snapshot does not show. Aggressive buyers lifting asks (buying at the ask price) suggest urgency. Passive buyers placing bids below the market suggest patience. The balance between aggressive and passive activity on each side often foreshadows short-term price direction.
Iceberg orders hide the true size of a large order by only showing a small portion in the book. When the visible portion is filled, another slice automatically appears. You can sometimes detect icebergs by noticing a bid or ask level that keeps refilling at the same price despite being hit repeatedly. This indicates a larger order is being worked behind the scenes.
The distribution of order sizes provides clues about who is trading. Retail orders tend to be smaller and appear at round number prices. Institutional orders are larger and may be split across multiple price levels using algorithms. Clusters of large orders at specific levels often represent significant players establishing or defending positions.
Time and sales data, also called the trade tape, complements the order book by showing every executed trade. While the order book shows intent, the tape shows what actually traded. Comparing the two tells you whether large orders in the book are actually being filled or just sitting there as decoration. Large trades that execute without corresponding visible book orders suggest dark pool or hidden order activity.
For practical trading, order book analysis is most useful for timing entries and exits. Before placing a large market order, check the depth at your expected execution price. If the book is thin, you might get significant slippage. Consider using limit orders or breaking your order into smaller pieces. Watch for order book imbalances before entering a position, as they can give you an edge on very short-term direction.