Out of book

A player is out of book when the position or latest move no longer belongs to the sequence that player knows or prepared for the opening, the first phase of the game. Book refers to published opening knowledge, even though that knowledge now also lives in databases, videos, and software. Up to that point the player may have recognized book moves; from then on, decisions must come mainly from the position on the board.

This is a relative condition, not an objective label for the move. One player may be out of book while the opponent still remembers analysis, and a large database may contain the position even when neither player knows it. A game can also leave opening theory with a good move, a doubtful move, or merely an uncommon one.

What changes in practice

Once memory ends, return to basic questions: what is the opponent threatening, which pieces remain undeveloped, whether either king is exposed, and which pawn exchanges could open lines. Clock management matters too, because trying to reconstruct a forgotten variation may waste time without solving the position in front of you.