ChessBase CTG opening-book format

An opening book is a collection indexed by positions rather than a linear list of games. For each position it can retain candidate moves, statistics, and preferences. Software consults this information early in a game and may choose a move before asking a chess engine, the program that analyzes positions, to perform a full search.

CTG is commonly expanded as ChessBase Opening Tree of Games. It is the opening-book format used by ChessBase and compatible interfaces. The book recognizes transpositions, meaning different move orders that reach the same position. A position can therefore be looked up once even when players arrived there by different routes.

Despite the name, a CTG book is normally not one self-contained file. ChessBase classifies files sharing the same base name with the extensions .ctg, .ctb, and .cto as the book's three components. Some implementations also use an auxiliary .ini file. All associated files should remain together when copied or backed up. A ChessBase CBH database, by contrast, stores games and related data rather than the book's position-based choices.