Book move

A book move belongs to the established body of knowledge about an opening. That knowledge, called opening theory, collects moves and plans studied through games and analysis. The expression comes from printed manuals, but the "book" may now be a database, a course, or another organized digital source.

The label means that the move has already been studied and played in comparable positions. It does not mean that the move is uniquely correct or necessarily best. Several book alternatives may exist, and an old recommendation may be displaced by newer games or more accurate analysis.

What depends on the reference

• A large database may recognize a rare move that a short manual omits.
• A move may be standard at one competitive level yet unfamiliar to most players.
• Two sources may evaluate the same continuation differently while both record it as theory.

When a game leaves the recorded continuations, commentators often say that it is out of book. This does not show that the new move is good or bad. It only marks the point where the chosen reference offers less established guidance.

In commentary and analysis, "book move" usually describes the state of chess knowledge, not what the player personally remembered. A player may find a book move independently, or memorize a theoretical move without understanding its underlying plan.