Direct answer
A forex order book is a structured view of outstanding buy and sell orders for a currency pair. It groups orders by price levels and shows the available sizes at each level. In principle, when buy and sell orders match, they execute (fully or partially), which removes or reduces orders in the book.
Because forex is often decentralized over the counter (OTC), what you see as an “order book” can depend on the specific venue or data source, so it may not represent all orders across the entire market.
How it works: the mechanics
Most order books reflect two sides:
- Bid side (buys): orders to buy the base currency.
- Ask side (sells): orders to sell the base currency.
Each side is typically organized by price levels. For each price level, the book shows quantity (how much is offered or requested at that price). A common term is top of book, meaning the best bid (highest buy price) and best ask (lowest sell price). The spread is the difference between those two.
As time passes, the book evolves through three main events:
- New order arrives: it adds size at a price level (or creates a new level).
- Order partially fills: matching executions reduce the order’s remaining size.
- Order cancels or expires: remaining size at that level is removed.
Example and independent checks
Consider a simplified snapshot for a pair:
- Best bid: buy 1.0 at 1.2500
- Best ask: sell 0.8 at 1.2502
- Next levels: other bids and asks at other prices
If a new sell order is posted at 1.2500 for 0.5, it can match against the best bid level first. After matching, the buy size at 1.2500 is reduced (from 1.0 to 0.5), while the new sell may be fully filled or may leave a remainder, depending on available opposing size.
Independent checks you can do without real-time assumptions:
- Verify that the best bid and best ask move when the book changes.
- Confirm that spread can widen or narrow as liquidity shifts between price levels.
- Compare how partial fills reduce displayed size rather than instantly removing entire levels.
Relevant limitations and risks
There are important limitations to interpret any forex order book correctly:
- Completeness risk: in OTC environments, an order book shown by one platform or feed may be incomplete relative to the full market.
- Venue differences: order types, matching rules, and update frequency can vary across venues and providers.
- Uncertainty: an order book is a snapshot of outstanding orders at a moment in time; changes between updates can be significant.
Because of these constraints, you should avoid treating a visible order book as a comprehensive map of all market liquidity or as a predictor of future outcomes.