A PolyGlot opening book is a collection of triples (Key, Move, Weight).

• Key is a 64-bit Zobrist hash key, of a Board position.
• Move is a move that can be made from that position.
• Weight is proportional to the probability the move should be played.

Castle

Enpassant

Extensions for other forms of Chess

File Format

Key

Learn

Move

Piece

Tests

Turn

Weight

http://hgm.nubati.net/book_format.html