Bids and asks
An order book lists resting buy and sell orders. Bids are prices buyers offer; asks are prices sellers request. The highest bid and lowest ask form the top of the book. The difference between them is the quoted spread.
If the best bid is $99.90 and the best ask is $100.10, the spread is $0.20. A market buy typically takes available asks; a market sell takes available bids. The last traded price is a different number and does not guarantee your next fill.
Depth tells you about size
Depth is the quantity available at each price level. A narrow spread can still sit above a shallow book. Check how much is offered near the top relative to the size you want to trade.
Suppose two units are offered at $100 and three at $101. Buying five units against those orders costs $503, giving an average price of $100.60 before fees. The best ask was $100, but there were not five units available there.
Resting orders can change
An order book is a live set of instructions. Orders can be added, cancelled or filled before your trade arrives. A large visible order is not a promise that liquidity will remain available.
Similarly, a price touching your limit does not guarantee your order fills. Earlier orders may have priority, and the amount actually traded at that level may be too small. Use order status and completed fills to determine what happened.
Connect the book to your order choice
Market orders prioritize access to available liquidity. Limit orders specify a price boundary but can remain unfilled. Post-only orders are intended to add liquidity and can be rejected if they would immediately take it.
In TRUE, inspect the market’s order controls and execution preview. A swap routed through liquidity pools is not necessarily represented by a conventional order book, so use the route and minimum-received information for that product instead.