How an order book actually works

An order book is a live, continuously updated list of the buy and sell orders that people have submitted for a particular market, sorted by price. On one side sit the bids — the prices buyers are willing to pay — and on the other the asks (also called offers) — the prices sellers are willing to accept. The order book is simply the record of everyone’s standing intentions, and it is the raw material every trade is built from.

The two sides of the book

Every order book has two halves. The bid side collects orders from people who want to buy, ranked from highest price at the top to lowest below. The ask side collects orders from people who want to sell, ranked from lowest price at the top to highest below. The best bid (highest price a buyer will pay) and the best ask (lowest price a seller will accept) meet in the middle, and the gap between them is the bid-ask spread.

Each line, or “level”, in the book records a price and the total quantity available at that price. If three different sellers all post an offer to sell at the same price, the book usually shows one level with their combined size. So a level is really a bucket: a price, and everything resting there waiting to be filled.

A worked example

Imagine a simplified book for a coin priced in dollars. These numbers are illustrative — they are here to show the structure, not to describe any real market:

Here the best bid is 100.0 and the best ask is 100.5, so the spread is 0.5. Nothing trades yet, because no buyer is willing to pay what any seller is asking. The book is in balance — a set of standing offers, waiting.

How orders enter the book and trades happen

When you place a limit order — an instruction to buy or sell at a specific price or better — you are adding a line to the book. If your price does not immediately match anything on the opposite side, your order rests there as visible liquidity, and everyone can see the size appear at your price level. This is what traders mean by “making” or “providing” liquidity: you are posting an offer others can hit later.

A market order behaves differently. Instead of resting, it says “fill me now at whatever the book offers.” It is matched immediately against the best available prices on the opposite side and removes liquidity rather than adding it. The mechanical differences between these two instructions are covered in market orders vs limit orders.

Using the example above, suppose a buyer sends a market order to buy 6 units. The matching system starts at the best ask, 100.5, and takes all 4 units there. It still needs 2 more, so it moves to the next level, 100.7, and takes 2 of the 9 units resting there. The order is now filled: 4 units at 100.5 and 2 units at 100.7. After the trade, the 100.5 level is gone entirely and the 100.7 level has 7 units left. The book has changed, and the new best ask is 100.7.

Notice that the buyer did not get a single price. Walking up through multiple levels to fill a larger order is exactly why the average price can differ from the price you first saw — the mechanism behind slippage. The deeper and more crowded the levels, the less an order has to walk. That cushion of resting size is what market depth measures.

Price-time priority

Within the book, orders are not filled at random. Almost all electronic venues use price-time priority: better prices are served first, and among orders at the same price, the one that arrived earliest is filled first. So if two sellers both offer at 100.5, the seller who posted earlier gets matched before the one who posted later. This simple, mechanical rule is what keeps the queue fair and predictable, and it is enforced by the matching engine that runs the market.

How the book updates, and how you see it

An order book is never still. Three kinds of events change it, and every trading interface is really just a visualisation of these updates arriving in sequence:

• Add: a new limit order arrives and either rests at a new or existing level, increasing the size shown there.
• Cancel: a participant withdraws a resting order, reducing the size at that level or removing the level entirely.
• Trade (fill): an incoming order matches resting size, removing that quantity from the book and printing a completed trade.

On busy markets these events can arrive thousands of times per second. What you see on screen is a snapshot reconstructed from that stream, which is why the top of the book can flicker and change even when no large trade is happening — participants are constantly adjusting where their orders rest.

Exchanges usually publish the book in tiers, and it helps to know which one you are looking at. “Level 1” data shows only the best bid and best ask with their sizes — enough to see the current market and spread. “Level 2” data shows several price levels deep on each side, which is what a depth view or market depth chart is built from. Some venues also offer a fuller feed that lists individual orders rather than aggregated levels. The deeper the tier, the more of the book’s structure you can see — but also the more noise, since much of that size can be cancelled at any moment. For most purposes, Level 2 is the sweet spot: enough to judge depth without drowning in individual-order churn.

What the order book does and does not tell you

The order book is a snapshot of stated intentions, and it changes constantly as orders are added, filled, or cancelled. It tells you where liquidity currently sits and how wide the spread is right now. It does not tell you what will happen next: resting orders can be cancelled in an instant, and large participants sometimes keep their real intentions off the visible book. Some venues also support hidden or “iceberg” orders, where only part of the true size is displayed.

It is also worth remembering that an order book is specific to one venue. Two exchanges each keep their own separate book for the same asset, which is one reason quoted prices can differ from place to place — the subject of why prices differ between exchanges.

What this means

An order book is not a prediction and not a scoreboard — it is a live ledger of who is willing to buy or sell, at what price, right now. Reading it means understanding four things: the best bid and ask define the current market, the spread between them is the cost of crossing, the size stacked at each level shows how much can trade before the price moves, and price-time priority decides who gets filled first. Everything else in market mechanics — spreads, slippage, depth, matching — is just a closer look at how this one structure behaves.