Understanding the Order Book
Trading Mastery
10 min read
Explore the limit order book structure, how resting orders create liquidity, and what the depth of market reveals about supply and demand.
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10 min read
Explore the limit order book structure, how resting orders create liquidity, and what the depth of market reveals about supply and demand.
An order book is the real-time list of every limit order waiting to be filled on an exchange — bids below the current price, asks above it. It's the X-ray of market intent: where people want to trade, at what prices, in what size.
The order book is like an X-ray of the market’s intent. It shows you where people want to buy and sell, at what prices, and in what sizes. For traders, it’s one of the most powerful tools available—but it’s often misunderstood or ignored.
But before you can read the order book correctly, you need to understand the types of orders that traders place. That’s what actually fills the order book—and what causes prices to move.
At root, every order is either a market order (execute now, accept the price) or a limit order (execute only at my price). Stops, icebergs, and post-only orders are variants built on top of these two — we'll meet them later in this module.
Example: You place a market buy order for 1 BTC. It instantly buys from the lowest available ask—say, at $64,000.
Example: You place a limit buy order for 1 BTC at $63,500. Your order sits in the book, waiting for someone to sell to you at that price.
Market vs limit orders: who takes liquidity, who provides it.
| Type | Execution | Role | Risk |
|---|---|---|---|
| Market Order | Immediate | Takes liquidity | Worse-than-expected fill |
| Limit Order | Delayed | Provides liquidity | Might never be filled |
Now that you know the two core order types, the order book will make much more sense.
The order book is a real-time list of all limit orders currently waiting to be filled on an exchange. It doesn’t show market orders—those get executed and disappear.
It contains:
Each level includes:
Here’s a simple order book snapshot:
| Price (USD) | Amount (BTC) | Type |
|---|---|---|
| 64,100 | 2.5 | Ask |
| 64,000 | 3.0 | Ask |
| 63,900 | 1.2 | Ask |
| 63,800 | ——— Mid ——— | |
| 63,700 | 1.5 | Bid |
| 63,600 | 2.8 | Bid |
| 63,500 | 3.5 | Bid |
Trades happen when a market order hits a limit order. At each price level, the exchange matches by price-time priority: the oldest resting order at the best price fills first. This is why being early to a level matters for makers — your queue position is a real edge.
Each time this happens, the trade moves price slightly in that direction. (We unpack this in detail in How Price Moves.)
Price doesn’t move because of predictions. It moves when someone chooses to buy or sell at a different price than the last trade.
The order book helps traders:
Example: If there’s a huge wall of buy orders at $63,000, that might act as a support level… unless those bids disappear before price reaches them.
Knowing what kind of orders move the market helps you:
On BTC, treat a spread wider than roughly 2 bps (about $1.30 at $65k) OR top-5-level depth under 5 BTC as a thin-book signal. Slippage on a market order at that point is likely to eat the edge.
But be cautious:
Drag the imbalance slider to see how bid/ask volume shifts create directional pressure. Notice how the ratio changes as one side of the book grows thicker.
An order book is the real-time list of every limit order waiting to be filled on an exchange — bids below the current price, asks above it. It does not show market orders, which execute and disappear immediately. The book is the X-ray of where traders want to buy and sell, at what prices, and in what size.
A bid is a buy limit order — someone willing to buy at a specified price or lower. An ask is a sell limit order — someone willing to sell at a specified price or higher. Bids sit below the current price, asks sit above it, and the gap between the best bid and best ask is the spread.
A market order executes immediately at the best available price and takes liquidity from the book; the trade-off is a worse-than-expected fill on thin books. A limit order executes only at your chosen price or better and provides liquidity by resting in the book; the trade-off is that it may never be filled.