An order book is the live list of every buy and sell order waiting to be filled on an exchange, organised by price. Buyers sit on one side, sellers on the other, and a trade happens when the two sides meet.
Behind every price you see quoted sits one of these. Reading it tells you something the price alone cannot: how much size the market can absorb before that price moves. For anyone trading Bitcoin or a thinly traded altcoin, that difference is the whole game.
The two sides
Bids are the buy orders. Asks, sometimes called offers, are the sell orders.
Bids are ranked highest first, because the buyer willing to pay most gets filled first. Asks are ranked lowest first, for the mirror reason. At the top of the book sit the best bid and the best ask, and the gap between them is the bid-ask spread.
Here is a simplified book for a token trading around $100.
| Side | Price | Size |
|---|---|---|
| Ask | $100.06 | 4,200 |
| Ask | $100.04 | 1,800 |
| Ask | $100.02 | 900 |
| Spread: $0.03 | ||
| Bid | $99.99 | 1,100 |
| Bid | $99.97 | 2,400 |
| Bid | $99.94 | 5,000 |
Buying 900 units at market takes the whole $100.02 level and fills at that price. A 3,000-unit order clears $100.02, then $100.04, and finishes partway through $100.06, giving an average price above the quote you started from. That difference is slippage, and the book is where it comes from.
How orders get filled
Two order types do most of the work.
Market orders execute immediately against whatever is resting in the book. Speed is guaranteed, price is not.
Limit orders specify a price and wait, and a stop-loss order is a variant that stays dormant until a trigger price is reached. They join the book at that level and sit there until someone trades against them, or until you cancel. Price is guaranteed, execution is not.
Matching almost always follows price-time priority. Better prices fill first, and among orders at the same price, whoever arrived first goes first. That second rule is why latency matters to professional traders, and why an exchange's matching engine speed is a real feature rather than a marketing line.
Makers and takers
Placing a limit order that rests in the book adds liquidity, which makes you a maker. Hitting an existing order removes liquidity, which makes you a taker.
Most exchanges charge takers more than makers, and some pay makers a rebate. The logic is straightforward: a book with nothing in it is useless, so the venue subsidises the people filling it.
Depth, and what it actually tells you
Market depth is the volume stacked at each price level. Two markets can quote an identical spread while one holds a hundred times more size behind it.
Usually depth is shown as a chart, with cumulative bid volume climbing to the left and ask volume to the right. A steep wall means large size concentrated near the current price. A shallow slope means the book thins out fast, and a modest order will travel a long way.
For anyone trading meaningful size, depth matters considerably more than the spread. A one-cent spread is irrelevant when there are only 200 units behind it.
What the order book does not tell you
- Resting orders can vanish. Anything in the book can be cancelled before it fills, and much of it is.
- Large orders hide. Iceberg orders display a fraction of their true size, so visible depth understates what is actually there.
- Spoofing exists. Illegal in regulated markets and still occurring, it means placing large orders with no intention of filling them, purely to influence other traders.
- It shows intent, not outcome. A wall of bids looks like support until it disappears the moment price approaches it.
- Depth is not stability. Market makers withdraw during stress, and a book that looked deep all year can empty in minutes.
When the book empties
That last point produced two of the more instructive events in recent crypto history.
On 21 October 2021, Bitcoin printed $8,200 on Binance.US while trading around $65,760 everywhere else, an 87% drop that reversed within roughly a minute. The exchange attributed it to a bug in an institutional client's trading algorithm, which fired enough sell orders to clear the entire bid side of that venue's book.
During the market-wide crash of 10 October 2025, market makers pulled their quotes across venues and books went effectively one-sided. Around $19 billion in leveraged positions were liquidated across 1.62 million accounts, roughly 87% of them longs. Several exchanges reported outages, and some users found their stop orders failed to execute at all.
Neither event reflected a change in Bitcoin's value. What both reflected is a book running out of orders on one side.
Order book vs AMM
| Order book | AMM | |
|---|---|---|
| Liquidity from | Traders and market makers posting orders | Deposited token pools |
| Price set by | Matched bids and asks | A formula and pool balances |
| Order types | Market, limit, stop, and others | Swap only |
| Available when quiet | Only if someone is quoting | Always, if the pool holds funds |
| Best suited to | Deep markets, precise execution | Long-tail tokens, small trades |
Both solve the same problem. An AMM replaces the counterparty with a formula, which works without anyone actively quoting. An order book keeps human intent visible, which is why traders read it.
Put this into practice on mb.io
Reading a book is one thing. Trading against a deep one, on an engine that matches quickly, is another.
mb.io is a regulated crypto spot exchange backed by MultiBank Group, a financial institution founded in 2005 that serves more than 2 million clients across 100+ countries.
- Regulated by VARA in the UAE and AUSTRAC in Australia
- 40-nanosecond execution speed
- 10/10 security score from Hacken, an independent blockchain security auditor
- Institutional-grade MPC custody powered by Fireblocks, with segregated client funds
- A curated list of assets, so you're not trading into thin books on tokens nobody supports
- 24/7 customer support, on web and on the iOS and Android apps
Open your account and start trading on mb.io.
Frequently asked questions
What is the difference between a bid and an ask?
A bid is a buy order and an ask is a sell order. At the top of the book sit the highest bid and lowest ask, and the gap between them is the spread.
What does order book depth mean?
The volume of orders waiting at each price level. Deep books absorb large trades with little price movement. Thin books move sharply, which is where slippage comes from.
Why do exchanges charge different maker and taker fees?
Makers add orders to the book and takers remove them. Since a book with nothing in it is useless, venues charge takers more and sometimes pay makers a rebate to keep it filled.
Can the order book be manipulated?
Yes. Spoofing, which means placing large orders with no intention of filling them to influence other traders, is illegal in regulated markets and still occurs. Iceberg orders also hide true size, legitimately.
What is a buy wall?
A large cluster of bids at one price level, which looks like it will hold the price up. Walls can be cancelled instantly, so they signal intent rather than a guaranteed floor.
Why did the price crash on one exchange but not others?
Because each venue runs its own book. When one book's bids get cleared, the price on that venue can collapse while the same asset trades normally elsewhere, as happened on Binance.US in October 2021.
Do decentralized exchanges have order books?
Most use liquidity pools instead, since posting and cancelling orders on-chain would be prohibitively expensive. Some newer platforms run order books off-chain and settle results on-chain.
