A list that is maintained by a specialist in order to track unfilled buy and sell orders. A specialist can enter clients’ orders as well as his own orders. The book is constantly monitored so that the specialist intervenes if needed to handle transient mismatches. Historically, specialists used notebooks before the advent of computer-based order tracking systems where they could observe orders in the book. However, nowadays investors off the exchange floor can also observe the book electronically.

It is also known as an order book.