Order Book
The live list of buy and sell orders for an asset, ordered by price — the primary display in every trading interface.
An order book lists outstanding buy orders (bids) and sell orders (asks) by price, with the quantity available at each level. The gap between the highest bid and lowest ask is the spread, and the accumulated quantity at each level is the depth — which together tell a trader what a given order size will actually cost, as opposed to what the last trade printed at.
As an interface it is one of the densest components in consumer software: numbers changing several times a second, colour carrying direction, and precision decisions that change what traders can see. The mistakes are consistent — animating every update so nothing can be read, using red and green as the only direction signal, and proportional figures that make columns impossible to scan. Price columns need tabular figures, updates need throttling to a readable rate, and direction needs a signal besides colour.
In practice
An exchange animated every order book row on change. At active moments the display was unreadable and traders reported “lag” that was not present in the data. Throttling updates to five per second with a brief highlight instead of a transition removed the complaint entirely and reduced CPU use on the page by about half.
Where teams get it wrong
- Animating every update, which makes the numbers unreadable exactly when they matter.
- Red and green as the only direction indicator.
- Proportional figures in price columns, so magnitudes cannot be scanned.
- Showing price levels without cumulative depth, which is what determines execution cost.
- Precision that changes between rows, making comparison harder than it needs to be.
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Related terms
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