Order Book
Category
Order-Mechanik
Sub-category
Order Book & Level 2
Curated by
Last reviewed
The order book is the price-sorted list of all open buy and sell orders for an instrument. It shows how much volume rests at which prices and thus determines spread, fill price and slippage. Traders read book depth and imbalances from it – for example whether far more volume sits on the buy side than on the sell side.
Context & Mechanics
Structure of the order book
The order book collects all resting limit orders for an instrument, sorted by price: bids on the buy side, asks on the sell side. The best buy limit and best sell limit together form the bid-ask spread. Level-1 data shows only these two prices; level 2, or the DOM (depth of market), also shows several price levels with their quoted volume – the book depth.
How orders move the book
Limit orders add liquidity to the book; market orders remove it by executing against the best opposite quotes, consuming one price level after another. Prices therefore do not move “because there are more buyers than sellers” – every transaction has both sides – but because aggressive orders exhaust one side's available liquidity faster and market makers adjust their quotes.
Book depth and slippage
How expensive a large order becomes depends on depth: the less volume rests near the current price, the more levels the order must walk through – the difference to the expected price is slippage. Checking depth belongs before any large order and in illiquid off-hours.
Limits of interpretation
The visible book is not the whole truth: iceberg orders display only part of their size, hidden liquidity and dark pools do not appear at all, and orders can be cancelled at any time. Futures and stocks have central order books; spot forex is fragmented – CFD traders see at most their provider's book. Order book imbalances are context, not an automatic signal. For prop traders, the book is above all a tool for understanding execution costs and placing entries in liquid zones.
Execution Example
The DOM of an index future shows asks of 40 contracts at 15,000.25, 35 at 15,000.50 and 60 at 15,000.75; bids of 85 contracts at 15,000.00 and 50 at 14,999.75. A trader wants to buy 100 contracts.
- A market buy order for 100 contracts consumes 40 at 15,000.25, 35 at 15,000.50 and 25 at 15,000.75.
- Average fill: 15,000.46 – 0.21 points above the best ask at order entry, purely from walking the book.
- New picture: the best ask is now 15,000.75 with 35 contracts remaining – price has risen “through the book” without any new information.
- Imbalance check: 85 contracts at the best bid versus 40 at the best ask looks buy-heavy, but visible size can be cancelled at any time or hide icebergs – context, not a signal.
Execution Risk & Errors
Mistaking visible depth for total liquidity – iceberg orders and hidden liquidity are missing
Reading book imbalances as reliable directional forecasts
Confusing level-1 data (best bid/ask only) with real market depth
Assuming a central order book in forex CFD trading even though the market is fragmented
Sending large orders without checking book depth first
Frequently Asked
What is the difference between level 1 and level 2?
Level 1 shows only the best bid and ask plus the last price. Level 2 (or the DOM) shows several price levels on both sides with the quoted volume – the book depth used to estimate execution costs.
Why does price move without any trades?
Bid and ask are quotes, not trades. When participants pull or move their limits, the best prices change without turnover – visible around news, when the book suddenly thins out.
Is there a central order book in forex?
No. Spot FX is organized decentrally across banks and ECNs. CFD and forex traders see at most their provider's book or an ECN excerpt – never the whole market.
How reliable are order book imbalances as a signal?
Only as context. Visible volume can be cancelled, icebergs show partial size and hidden liquidity is missing entirely. A heavier side does not guarantee direction.