An order book is the live list of buy and sell orders for a market, ranked by price. It shows who wants to buy, who wants to sell and how much is available at each price level.
At the top sit the best prices currently available. The highest bid is the best price a buyer will pay. The lowest offer, often called the ask, is the best price a seller will accept. Together they form the visible spread. The order book updates as new orders arrive, get cancelled or are filled.
What an order book shows: bids, offers and depth
The book has two sides. Bids are buy orders, sorted from highest price down. Offers are sell orders, sorted from lowest price up. Each entry has a price and a size, sometimes shown as multiple orders queued at the same price. The best bid and best offer are called the top of book. Levels below them show depth, which is the extra liquidity available once the top level has been traded.
A simple snapshot might look like this:
| Bid size | Bid price | Ask price | Ask size |
|---|---|---|---|
| 1,200 | 100.00 | 100.02 | 800 |
| 900 | 99.98 | 100.04 | 1,500 |
| 1,600 | 99.96 | 100.06 | 600 |
From this you can read the spread, which is 0.02, and how much size might be filled before the next price level comes into play.
How orders match: price–time priority in practice
Most venues use price and then time to decide who trades first. Better prices go first. If two orders are at the same price, the order that arrived earlier sits ahead in the queue. Some futures and options markets may use pro rata or other rules. Exact matching depends on the venue and the instrument.
Resting orders in the book are usually created with a limit order. An order that is willing to trade now, such as a market order or a limit that crosses the spread, executes against the resting orders starting at the best price and walking the book if the size is larger than what is available at that level.
Example. Using the snapshot above, a buy market order for 1,500 shares would fill 800 at 100.02 and 700 at 100.04. The volume-weighted average fill price would be (800 × 100.02 + 700 × 100.04) ÷ 1,500, which is 100.029. If the mid price before the trade was 100.01, the slippage on this order would be about 0.019 per share. After the trade, the new best offer would be whatever remains at 100.04.
Partial fills are common. If your limit price cannot reach the other side, your order will rest until a matching order appears or you cancel. If you improve the price, you can jump the queue at that new level. If you amend size without changing price, your original queue time often stays, though rules can vary by venue.
Reading the book: spread, depth and imbalance
Traders scan the spread to judge immediate trading cost. A tight spread is usually a sign of healthy competition between buyers and sellers. A wide spread often points to poor liquidity or higher uncertainty. Depth shows how much you can trade before the price is likely to move. If depth vanishes after the top level, large orders are more likely to push the price.
Imbalance is the difference between visible buy and sell size near the top. A heavy bid side can hint at short term support. A stacked offer side can cap rallies. These signals are noisy. Orders can be cancelled in a flash, and some venues allow hidden or iceberg orders that do not display full size. Book reading works best as context for price action and volume, not as a stand-alone system.
Remember that the order book only shows willing prices. It does not guarantee execution at those levels if size changes before your order arrives. Fast conditions can empty a level between ticks, which is one reason slippage occurs.
Auctions, hidden size and other venue quirks
Electronic markets run two main phases. Auctions at the open and close, and continuous trading in between. During an auction, orders collect without matching until a single price clears the most volume. Some platforms publish an indicative match price and imbalance during the call. After the auction uncrosses, the order book resumes continuous matching.
Many venues offer advanced order types. Iceberg orders display only a tranche of the full size. Hidden orders do not display but can still provide liquidity when prices cross. Pegged orders follow a reference such as the mid price or the best bid. The exact behaviour, queue priority and availability of these features vary by exchange or broker.
Some markets run maker taker fee schedules. Posting liquidity can earn a rebate, while taking liquidity pays a fee. That can affect how participants decide to rest orders or cross the spread. In other markets, fees are symmetrical. Pricing varies by provider and can change.
Fragmentation matters too. A single stock or crypto asset may trade on several venues at once. That means no single book shows all liquidity. Professional tools use smart routers to scan multiple books and seek the best combination of price and size. In some jurisdictions there is a consolidated view of top quotes. In others you must piece it together from several feeds.
Where you see it and why it matters for execution
Order books appear on most exchange and broker platforms. Level 1 data shows only the best bid and best offer with the last trade. Level 2, often called market depth, shows multiple price levels and sometimes the queue size at each level. Access to full depth and the number of levels you can see depends on your data plan and provider.
For many equity and crypto trades, your broker routes orders either into the central book or to a market maker. Some deals are negotiated off book then reported. That is common for large blocks where the participants prefer not to show their full size in the visible book.
Why it matters. The order book sets your likely execution price and the risk of slippage. If you post a limit inside the spread, you might improve the price and get filled as someone crosses. If you must trade now, a marketable order will sweep the top levels. Watching depth helps you judge how much size you can send without moving the market more than you intend.
Common confusion arises around cancellations. Seeing the offer side thin out before a rally does not prove intent. Many cancellations are routine risk control. Conversely, a thick book can vanish when volatility rises. Treat what you see as transient. It is a queue, not a promise.
In short, the order book is the venue’s live schedule of who wants to trade and at what price. It is the raw material behind quotes, spreads and fills, and knowing how it works helps you choose between posting a patient limit and sending an aggressive order when timing matters.