Opening book

An opening book is a prepared collection of early-game positions and moves associated with them. Some chess programs consult one before starting the normal search of a chess engine, meaning before calculating through different possible continuations. If the current position is in the book, the program can select a stored move without having to discover it from scratch.

A book can be built from game databases, analysis, or manual selection. Entries may contain several alternatives with information such as frequency, results, or weights, numbers that raise or lower the chance that each move will be chosen. Once an opponent reaches a position the book does not contain, the program is informally said to be out of book and continues with normal search.

A book does not prove that a move is objectively perfect; its quality depends on how the material was created and maintained. It is also different from a tablebase, which exactly solves covered endgames, and from a transposition table, which stores results generated during the current search.