An order book is the live, price-ranked list of buy and sell orders resting in a market. It shows where displayed buyers are bidding, where displayed sellers are offering, and how much visible size is available at each price level.
The order book is useful because it connects a quoted price to executable market depth. It does not prove that all demand or supply is visible, and it is not a valuation model or a trading recommendation.
Key Takeaways
- The best bid and best ask at the top of the book form the visible inside market and help determine the Bid-Ask Spread.
- Depth beyond the best quote helps traders estimate whether a larger order may cause slippage.
- Displayed orders can be canceled, repriced, hidden, or split across venues, so the book is useful evidence but not a guarantee of execution.
How an Order Book Works
Most electronic order books have two sides:
- Bids: prices where buyers are willing to buy.
- Asks or offers: prices where sellers are willing to sell.
The highest bid and lowest ask are the top of the book. A limit order can add displayed liquidity to the book. A market order usually removes liquidity by trading against the best available opposite-side orders.
Order priority rules vary by market and venue, but price is usually the first sorting rule. Time priority, market-maker obligations, hidden order handling, and auction rules can also affect which orders trade first.
| Data view | What it shows | What it misses |
|---|
| Order book | Displayed resting bids and offers by price level | Hidden liquidity, future cancellations, off-book interest |
| Trade tape | Completed trades | Unfilled orders and current depth |
| Best bid and offer | Top visible buy and sell prices | Depth away from the best prices |
| Market-depth feed | More levels of displayed depth | Orders not displayed on that venue or feed |
Common Mistakes
- Treating displayed size as guaranteed liquidity: orders can be canceled before they trade.
- Reading one venue as the whole market: liquidity may be fragmented across exchanges, alternative trading systems, dealers, and dark venues.
- Ignoring order size: the same book can look liquid for
100 shares and thin for 25,000 shares. - Confusing microstructure with value: a thick book can make trading easier, but it does not make the asset fairly valued.
- Using stale screenshots: order-book evidence is timestamp-sensitive.
How To Evaluate Order-Book Evidence
When the order book affects a trading or liquidity decision, check:
- timestamp and data latency
- venue or consolidated feed coverage
- best bid, best ask, and spread
- displayed size at and beyond the best prices
- recent trade prints and Trading Volume
- order type, order size, and time-in-force
- whether the market is in regular hours, an auction, or a stressed period
This page is educational only. It explains market mechanics and should not be treated as personalized trading, investment, legal, or regulatory advice.
Sources and Further Reading
FAQs
Does the order book show every participant's true buying and selling intention?
No. It shows displayed orders, not the full hidden or future liquidity in the market.
Why do traders care about order-book depth?
Because depth affects execution quality, slippage, and how much a trade may move the market.
Can the order book look strong and still fail to hold a price level?
Yes. Visible orders can be canceled or overwhelmed quickly, especially in fast or thin markets.
- Bid-Ask Spread: The gap between the best displayed buy and sell prices.
- Liquidity: The broader concept of trading quickly without a large price concession.
- Market Depth: The available visible size across multiple price levels.
- Market Maker: A participant that often helps supply two-sided quotes.
- Limit Order Book: A book built from resting limit orders.