What is a Limit Order Book (LOB)? The Architecture of Price Discovery

The Limit Order Book is the digital ledger of all buy and sell orders. Discover how price-time priority and order depth dictate market movements.

After debugging a C++ parser for NASDAQ's ITCH feed for 16 hours straight, I saw how the Limit Order Book (LOB) collapses during a flash crash. The LOB is the transparent, real-time list of all outstanding limit orders on an exchange, organized by price level.

Each level in the book shows the total volume available at that specific price. When a market order arrives, it 'walks the book,' consuming the available volume at the best prices until the order is filled. The cumulative depth of the book at price $P$ is the sum of all orders $q_i$ such that $P_i \ge P$ for asks, or $P_i \le P$ for bids: $Depth(P) = \sum q_i$.

The LOB is managed by a matching engine that follows a strict Price-Time Priority protocol. This means the best price always executes first, and if two orders share a price, the one that arrived earlier takes precedence.

1. Limit Orders: Orders to buy or sell at a specific price. These provide liquidity and 'make' the book. 2. Market Orders: Orders to buy or sell immediately at the best available price. These consume liquidity and 'take' from the book. 3. Cancellations: Participants can remove their limit orders at any time, which happens at microsecond scales in HFT.

The depth chart allows quants to visualize where large blocks of liquidity are sitting, which often act as support or resistance levels.