Order book basics

Bids, asks, spread, depth, slippage — and why your available balance can look low.

The vocabulary

Term	What it means
Bid	An order offering to buy shares at a price.
Ask	An order offering to sell shares at a price.
Best bid	The highest current buy price.
Best ask	The lowest current sell price.
Spread	The gap between the best bid and the best ask.
Depth	How many shares are available at each price level.
Liquidity	How easily a position can trade without walking through many price levels.
Slippage	The difference between the first displayed price and your average execution price across the filled size.

Reading depth before you size an order

The best price applies only to the shares available at that price. A larger order walks up through worse levels, and your average price is what you actually pay.

Before sizing up, look at how much depth sits within a price range you’d accept. If the book thins out two levels in, that is your real capacity — not the headline quote.

Reserved cash and shares

Resting orders hold funds:

• an open buy reserves cash equal to its cost
• an open sell reserves the shares it would deliver

Reserved amounts are unavailable for anything else until the order fills or you cancel it. If a new buy is rejected for insufficient funds while your wallet shows a balance, check open orders first.

Orders can be cancelled for you

A resting order can be cancelled without you asking. A paused market rejects new orders but keeps existing ones, ready to return to Active. Resolved, Voided and Closed are terminal and cancel resting orders.

There are other conditions under which the book cancels resting orders — integrators should read Auto-cancellation.

Resolved and voided positions can become claimable. See How markets resolve.