An order book is the live ledger of unexecuted buy and sell interest at a trading venue.
An order book is the live ledger of unexecuted buy and sell interest at a trading venue. It lists resting bids and offers by price level and updates as participants add, change, cancel, or execute orders. For market microstructure work, it is the venue-level state that shows currently displayed supply and demand.
At its simplest, the data structure is organized around three core fields: price, size, and side. Price is the level at which an order is willing to trade. Size is the displayed quantity resting at that level. Side marks whether the order is a bid, which is buy interest, or an offer or ask, which is sell interest. In practice, order-book datasets often include additional identifiers and timestamps, but the basic ledger is the mapping from side and price to resting quantity.
Because the book is a ledger rather than a single quote, multiple price levels can be visible at once. The highest bid and the lowest offer form the best displayed prices. Deeper levels show additional liquidity farther from the current trading point. A snapshot of the book at any instant is therefore a table of resting quantity across price levels on both sides.
The mechanism is event driven. When a new limit order arrives and does not immediately trade, it is added to the ledger at its price level. When an existing order changes in size or price, the resting quantity at the relevant level is modified. When an order is withdrawn, its quantity is removed from the ledger. When an incoming order trades against resting liquidity, the executed quantity is deducted from the book, and levels can disappear or move if their displayed quantity is exhausted. These add, modify, cancel, and execute events are what turn the order book into a live state variable rather than a static table.
This event-by-event view matters in fragmented electronic markets. Sonar’s cross-venue data material describes venue-synchronized market data and the problem of comparing activity across exchanges with different feeds and timestamps. That framing is consistent with treating each venue’s order book as its own live ledger, where snapshots and updates must be aligned carefully before researchers compare displayed liquidity across venues. The same source also illustrates venue-level market data with synchronized displays, which is the practical setting in which real-time order-book snapshots are examined.
For quantitative research, the order book is useful because it separates displayed interest from completed trades. Trades tell you what has executed. The book tells you what is currently available to execute against, subject to cancellation and queue priority. That distinction is central to studying liquidity, spread formation, queue dynamics, and the short-horizon response of prices to changes in displayed depth.
A key caution is that the order book is only the displayed ledger at a venue. It does not by itself reveal hidden liquidity, off-venue interest, or whether resting orders will remain in place long enough to trade. It is a real-time picture of visible supply and demand, but it is conditional on the venue, the feed, and the exact sequence of events used to reconstruct the state.
Covered in depth in the Cross-venue market data & signals pillar hub.