When I open an order book, I check one thing first: which venue and data feed am I looking at?
An order book is a live list of visible buy and sell orders. It shows quoted prices and the quantities currently available through a specific exchange, broker, ECN or feed. It is not the whole market.
That makes the order book useful for a practical question:
If I send an order now, where could it trade and how far might it move through the available liquidity?
It cannot tell me where price will go next. But it can explain spread, depth, partial fills and slippage before they appear in an execution report.
Order book definition
An order book records resting orders for one instrument and arranges them by price.
- Bids are orders to buy. The highest bid comes first.
- Asks, also called offers, are orders to sell. The lowest ask comes first.
- Quantity shows how much is displayed at each price.
- Best bid is the highest displayed buying price.
- Best ask is the lowest displayed selling price.
- Spread is the difference between the best ask and best bid.
Consider this simplified book:
| Side | Price | Quantity |
|---|---|---|
| Ask | $101.80 | 300 |
| Ask | $101.60 | 450 |
| Ask | $101.40 | 600 |
| Bid | $101.20 | 500 |
| Bid | $101.00 | 350 |
| Bid | $100.80 | 400 |
The best bid is $101.20. The best ask is $101.40. The spread is $0.20, and the midpoint between them is $101.30.
No trade has happened just because these orders are visible. The highest buyer is still below the lowest seller. A trade needs a new order that accepts an available price, or an existing order must be changed so the two sides cross.
How to read an order book
I read a book from the inside out.
The prices nearest the spread matter first because they are next in line for execution. Prices farther away show additional depth if a larger order consumes the first level.
In the example, a small market buy can execute at $101.40. A larger buy may use all 600 units at that price and continue to $101.60. This is why the best ask is not a promise that an entire order will fill there. It is only the best displayed price for the quantity currently available.
For a sell, the order meets the highest bids first and moves down if it needs more quantity.
The bid-ask spread is one part of the execution cost. Depth is the other. A market can show a narrow spread but very little size near the top of the book. It may look liquid until a meaningful order arrives.
What happens when a market order hits the book
Suppose I send a market order to buy 700 units.
The matching engine starts with the lowest ask:
- It buys 600 at $101.40.
- The order still needs 100.
- It buys the remaining 100 at $101.60.
The total cost is:
(600 × $101.40) + (100 × $101.60) = $71,000
The average fill price is:
$71,000 ÷ 700 = $101.4286
If all 700 units had been available at the original best ask, they would have cost $70,980. Moving into the next level added $20 before fees. That difference is the order’s slippage against the price visible when it arrived.
After the trade, the $101.40 ask is gone. The quantity at $101.60 falls from 450 to 350. If no new orders arrive, the best ask becomes $101.60 and the spread widens to $0.40.
What happens when a market buy hits the book
A 700-unit order takes all 600 units at the best ask, then moves to the next price for the final 100.
600 at $101.40
100 at $101.60
(600 × $101.40) + (100 × $101.60) = $71,000
A market order prioritizes execution, not a fixed price. A conventional market order consumes resting liquidity and does not remain open in the book. Exchanges may apply price protections or specific rules to any unfilled quantity, so the exact handling depends on the venue.
How limit orders enter the queue
Now suppose I place a limit order to buy 200 units at $101.20.
It does not cross the best ask at $101.40, so it can rest on the bid side. The book already shows 500 units at $101.20. On a venue that uses price-time priority, my order normally joins behind orders already waiting at that price.
Price comes first. Time breaks the tie.
If sellers later reach $101.20, the orders ahead of mine may fill before mine does. Seeing my limit price trade is therefore not proof that my full order should have executed. Queue position matters.
A limit order can also be marketable. For example, a buy limit at $101.60 crosses the asks at $101.40 and $101.60. It can execute immediately but cannot pay more than $101.60.
This gives the order a chance to trade now without accepting a price beyond the cap. The time-in-force controls any remainder:
- GTC or day order: the remainder may rest in the book at the limit price.
- IOC: any quantity not filled immediately is cancelled.
- FOK: the order fills in full under the venue’s rules or not at all.
Order names and behavior can vary. I always check the rules of the platform I am using.
How the matching engine chooses an order
The matching engine applies the venue’s rulebook to incoming orders.
Price-time priority is common:
- The best eligible price trades first.
- At the same price, the oldest eligible order trades first.
- If the incoming quantity remains, the engine continues through the queue or moves to the next eligible price.
But price-time is not universal. Some futures markets use pro-rata or hybrid allocation, where several resting orders at the same price receive part of an incoming order based on size or another formula.
How an incoming order moves through the matching engine
The route changes depending on whether the order can trade at a price already available on the opposite side.
This is also where the terms maker and taker come from. A resting order adds liquidity and may be treated as a maker order. An incoming order that matches it removes liquidity and may be treated as a taker order. Fees or rebates depend on the venue and account type.
Market depth and slippage
Market depth is the quantity available across multiple price levels. It matters most when the order is large relative to the size near the best price.
Using the same book:
| Market buy size | Fill | Average price | Cost above the initial best ask |
|---|---|---|---|
| 100 | 100 at $101.40 | $101.4000 | $0 |
| 700 | 600 at $101.40, 100 at $101.60 | $101.4286 | $20 |
| 1,100 | 600 at $101.40, 450 at $101.60, 50 at $101.80 | $101.5000 | $110 |
This is a snapshot, not a quote. Other participants can trade, add or cancel orders while mine travels to the venue. I still calculate cumulative quantity across the prices I may need. It gives me a more realistic cost estimate than the best bid or ask alone.
Order book vs trade history vs depth chart
These three displays describe different things.
| Display | What it shows | What it does not show |
|---|---|---|
| Order book | Visible resting bids and asks | Completed trades |
| Trade history or tape | Prices and quantities that actually traded | Orders still waiting |
| Depth chart | A visual, cumulative view of book quantity by price | A separate source of market activity |
This distinction matters. The book shows stated interest. The tape shows completed decisions.
If a large bid appears and disappears without trading, it did not absorb selling. If trades repeatedly execute into that bid and the displayed quantity keeps returning, that tells me something different. I prefer to read changes in the book alongside actual executions rather than treat either one as a complete signal.
Market by price, market by order and data levels
Not every feed shows the same detail.
Market by price aggregates all displayed quantity at each price level. The 500-unit bid at $101.20 may represent one order or dozens of orders.
Market by order shows individual anonymous orders where the venue makes that data available. It can reveal queue composition and changes that an aggregated feed hides.
Platforms also use labels such as Level 1, Level 2 and Level 3:
- Level 1 usually means the best bid, best ask and their displayed sizes.
- Level 2 usually adds multiple price levels or venue detail.
- Level 3 may include individual order data or order-management access.
These labels vary by market and vendor. The feed specification is more reliable than the label.
What the order book cannot tell you
The visible book has several blind spots.
It may cover only one venue. A US stock can trade across many exchanges and off-exchange venues. One exchange book is not the entire US market.
Some liquidity is hidden. Iceberg orders display only part of their size. Non-displayed orders and dark venues do not appear as ordinary price levels.
Orders can be cancelled. A displayed bid is an option to trade while it remains active, not a commitment to defend that price.
It does not show future market orders. A book may look balanced just before a large buyer or seller arrives.
The feed has latency. What I see on screen may already be behind the venue’s current state.
For these reasons, a large visible bid is not automatically support. A large ask is not automatically resistance. Order imbalance can describe the current display, but by itself it is a weak forecast.
Spoofing makes this limitation more serious. A trader may place orders intended to create a false impression of supply or demand and cancel them before execution. Ordinary order cancellation is not automatically spoofing; intent and the pattern of conduct matter. But the possibility is another reason not to trade from a single “wall.”
How order books differ across markets
The basic bid-and-ask logic travels well. The market structure does not.
Stocks
Stocks trade across exchanges and other venues. In the US, the national best bid and offer combines the best displayed prices from protected venues. An individual exchange book shows only that exchange.
Futures
Exchange-traded futures usually use a central limit order book for each contract on a venue. The matching algorithm can differ by product. Price-time, pro-rata and hybrid rules all exist.
Forex and CFDs
There is no single global spot FX order book. A broker, bank, ECN or liquidity provider shows the pool available through its own network. A CFD platform may display broker-generated or aggregated liquidity rather than a central exchange book. Two platforms can therefore show different depth for the same currency pair.
Crypto
A centralized crypto exchange maintains its own book, so price and liquidity can differ across exchanges. Decentralized venues may use limit orders, but many use automated market makers. An AMM pool is not a conventional order book.
The checks I make before using order book data
I do not use the book as a buy or sell button. I use it to test the execution conditions around a decision already made.
- Identify the venue and feed. I want to know which part of the market I can actually see.
- Check the spread. A wider spread raises the immediate cost of demanding liquidity.
- Compare my size with displayed depth. The best price matters only for the quantity available there.
- Estimate the average fill. I add the levels an order may consume.
- Watch completed trades. I want to see whether displayed liquidity is actually trading, cancelling or replenishing.
- Choose the order type deliberately. Market orders prioritize execution. Limit orders set a price boundary but may not fill.
- Account for fees and latency. The displayed book is not the final execution report.
This process does not predict the next move. It reduces avoidable surprises between the decision to trade and the price received.
Use the book to plan execution, not predict price
An order book is most useful when I ask it a narrow question. Not “where will the market go?” but “what liquidity is visible, what is ahead of my order, and what could this trade cost to execute?”
That is enough. Used this way, the book turns a quoted price into a clearer picture of spread, depth and execution risk.
