What Is an Order Book? Bids, Asks and Spread Explained
An order book is a live list of available buy and sell instructions for a market. Bids show prices buyers are willing to pay; asks show prices sellers are willing to accept. Reading the top levels helps explain the current spread, but it does not guarantee the price of a larger trade.
Start with the best bid and best ask
The highest available bid is the best bid. The lowest available ask is the best ask. The difference between them is the bid-ask spread. Coinbase's order-book introduction uses the same definitions and explains how depth charts summarize cumulative orders.
Consider this hypothetical BTC/USDT book:
| Side | Price | BTC available |
|---|---|---|
| Ask | 50,020 | 0.004 |
| Ask | 50,010 | 0.003 |
| Bid | 50,000 | 0.005 |
| Bid | 49,990 | 0.010 |
The best bid is 50,000 and the best ask is 50,010, so the spread is 10 USDT per BTC. This does not mean every order will execute at the midpoint or even at one price.
A market order can consume several levels
Suppose a market buy seeks 0.005 BTC. It first reaches the lowest ask: 0.003 BTC at 50,010. The remaining 0.002 BTC then reaches the next ask at 50,020.
Gross quote value = (0.003 × 50,010) + (0.002 × 50,020) = 250.07 USDT.
The weighted average execution price is 250.07 ÷ 0.005 = 50,014 USDT per BTC before fees. The order did not receive the initial best ask for its entire quantity because only 0.003 BTC was available there. All prices and quantities are invented examples.
This difference between an expected reference price and the actual average fill is one form of slippage. The book can also change between observation and execution.
Limit orders set a boundary, not a completion promise
A limit buy sets the highest price the trader is willing to pay. A limit sell sets the lowest acceptable price. If the instruction cannot match within that boundary, it may remain open or fill only in part.
A buy limit at 50,000 in the example would not immediately match the 50,010 best ask. It could join the bid side, subject to the venue's matching rules. If another order already waits at the same price, time or other priority rules can affect which instruction fills first.
A limit order can still execute immediately if its price crosses available orders. “Limit” therefore does not automatically mean maker, and “market” does not promise one displayed price.
Depth is specific to the market
BTC/USDT on one venue has a different book from BTC/USDT elsewhere. BTC/USDC is also a different market. Liquidity cannot be inferred from the asset name alone.
Visible quantities may change through new orders, cancellations and executions. A large displayed wall can disappear; it is not proof of future support or resistance. Some interfaces aggregate multiple prices into one visible row, so the selected precision can change how depth appears.
The recent-trades panel is not the order book. It records completed matches, while the book shows currently available instructions. Likewise, the last traded price can sit outside an earlier spread after the market changes.
A practical reading checklist
Before placing an order, confirm the product and pair, then identify the best bid, best ask, spread and quantity available near the intended size. Read whether the form asks for base quantity or quote value. After submission, verify filled quantity, average price, fees and any unfilled remainder.
OrangeX.com's spot trading guide explains where to select a pair and choose between Market and Limit orders. Interface details and available markets can change.
Common questions
Is the middle of the spread a guaranteed execution price?
No. Execution depends on available opposing orders and the instruction used.
Does a deep order book eliminate slippage?
No. Greater nearby depth can reduce price impact for a given size, but orders can change and fees still apply.
Is every visible order guaranteed to remain?
No. Unfilled instructions may be changed or cancelled under the venue's rules.
Practice calculating the required depth before using a live order, and review related concepts in OrangeX.com Academy.
