An order book is the live list of all outstanding buy and sell orders and quotes for one financial instrument, says the trading firm Optiver. Market makers and other participants post them. In the words of the data blog CryptoDataDownload, it is "a list of willing buyers and sellers and the price(s) at which they are willing to conduct a trade." A trade happens when a buy order and a sell order meet at the same price. A market order takes the best price already in the book. A limit order that does not fill at once waits in it. The same structure is also called a Central Limit Order Book, or CLOB.
Key facts, per Optiver
- Matching rule: Price-time priority decides the fill order on most order books.
- Same price: The order that arrived first is first in line.
- Spread: The lowest asking price minus the best bid.
- Liquidity: Judged from the volume on the bid and ask sides.
How orders stack and trades form
CryptoDataDownload describes two stacked lists. Buy limit orders run from the highest price down to the lowest. Sell limit orders run the other way, from the lowest price up. Optiver explains what happens where they meet. A buy and a sell at the same price produce a trade. Most order books rank waiting orders by price first and arrival time second, a rule called price-time priority. When two orders share a price, the one that arrived first fills first.
Bid, ask, spread and depth
CryptoDataDownload defines the bid as the most an investor will pay for a security. The ask is the least a seller will accept. Optiver gives the spread as one subtraction: the lowest ask minus the best bid. It says traders can judge liquidity from the volume waiting on each side. The cryptoanalysis.ai explainer adds market depth. Depth is how much volume rests at every price level across the whole book, not only at the best bid or ask.
Market orders and limit orders
Optiver describes market and limit orders among several common order types. A market order buys or sells at the best price currently available in the book. It normally fills, but the price received is not guaranteed. A limit order sets a maximum price to pay or a minimum price to receive. Its execution is not assured. If it does not fill at once, it joins the order book and waits.
Market order vs limit order
Which risk does each order type leave you with: an uncertain price or an uncertain fill?
| Order type | Price | Execution | Place in the order book | Source |
|---|---|---|---|---|
| Market order | The best price currently available in the order book | Normally ensures execution, but not the price received | Takes an existing price from the book | Source: Orders and the order book - Optiver (optiver.com), accessed Oct 14, 2026 |
| Limit order | A maximum price to pay or a minimum price to receive | Not assured | Appears in the book if not executed immediately | Source: Orders and the order book - Optiver (optiver.com), accessed Oct 14, 2026 |
These are Optiver's general definitions; order options and rules differ by platform, and the table does not predict fill prices.
Who keeps the book full
Optiver defines a market maker, also called a liquidity provider, as a firm or person that continuously quotes both sides. That means bids to buy and offers to sell, as a primary trading strategy. The cryptoanalysis.ai explainer describes how that supply shifts. In calm markets, market makers compete with tight spreads and thick order books. In volatile periods, they widen their quotes or pull resting orders. A separate explainer covers what a liquidity provider does in different markets.
Why the price you see can change
The cryptoanalysis.ai explainer defines slippage, and flags that resting orders can be cancelled at any moment. Slippage is the gap between the displayed price when the order goes in and the average price it fills at. Because resting orders can disappear, the book shows "intent, not commitment."
What the screen shows depends on the data level
CoinAPI, a market-data vendor, sorts order book data into levels. Level 1 shows only the best bid and ask prices and their volumes. Level 3 shows every individual order with a unique identifier and a timestamp. It tracks each order from placement to execution or cancellation. That detail exists only where a venue publishes per-order data.
Analysis: each order type picks a risk
Optiver and cryptoanalysis.ai, read together, point to one practical choice. A market order usually secures the fill but not the price. A limit order fixes the worst price but not the fill. The book behind both is less firm than it looks. Orders in it can be cancelled at any moment. Market makers widen quotes or pull orders when markets turn volatile. That suggests the visible book may be thinnest just when a trader most needs a fill.
What is confirmed and what is not
Optiver and CryptoDataDownload give the same core definition: a list of outstanding buy and sell orders and their prices. Most other points rest on one outlet. The matching rule, price-time priority, comes from Optiver alone. Optiver defines market and limit orders, and CryptoDataDownload also describes both types. CryptoDataDownload alone describes how the two sides are stacked. The cryptoanalysis.ai explainer alone covers depth, slippage and market-maker behavior under stress. CoinAPI, which sells this data, alone describes the levels. Its glossary also gives cancellation figures with no source, so they are not used. None of the four gives a dated cancellation rate. None of the four states a publication date in its text. None says how far spreads widen under stress, or which venues publish per-order data.
The order book lists all outstanding orders and quotes in a particular financial instrument posted by market makers and other market participants.
What to check before your next order
The choice between a market order and a limit order is a choice of risk. Before placing an order, check which uncertainty it leaves. A market order normally fills, but the price is not guaranteed. A limit order caps the price, but its execution is not assured. Order options and rules differ by platform, so read the platform's own order-type help before you trade.
Sources: Orders and the order book - Optiver (optiver.com), accessed Oct 14, 2026 · Understanding the Orderbook (cryptodatadownload.com), accessed Oct 14, 2026 · Crypto Order Book Explained: Depth, Spreads, and Slippage (cryptoanalysis.ai), accessed Oct 14, 2026 · CoinAPI.io Glossary - Order Book Levels (coinapi.io), accessed Oct 14, 2026
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