PolyGlot opening book format

An opening book is a database of prepared moves for early-game positions. Instead of calculating from scratch, software can look up the position and choose one of the stored continuations. The PolyGlot format is a widely supported way to store that book in a binary file, commonly using the .bin extension.

What each entry stores

A PolyGlot book is an ordered sequence of 16-byte records. Each record contains four values:

• A 64-bit key that acts as a numerical fingerprint of the position through a method called Zobrist hashing.
• A move encoded in 16 bits.
• A 16-bit weight that can influence how often the move is selected.
• A 32-bit learn field reserved for learning information or application-specific use.

The key makes it fast to find all moves associated with a position. The weight is not a universal evaluation and does not prove that a move is best. It expresses a preference determined by how the book was built and how the reading program interprets the data.

PolyGlot is also the name of a program that can adapt UCI engines to CECP interfaces. The adapter and the book format are historically related, but they are not the same thing.