Out of book
A player is out of book when the game reaches a position that neither side is recalling from opening knowledge or preparation, so from that move on the choices have to be worked out at the board.
Also called: leaving book
Being out of book is not a mistake in itself. It simply marks the point where memory stops and calculation and judgment start. For a well prepared player that point may come very late, after twenty moves or more. For a player who learned only the first few moves of an opening it can come on move five.
The moment matters because the clock and the position change character together. Until then, moves cost almost no thinking time. Afterwards, every move needs a plan, and a player who arrives there with a sound structure and a clear idea of what the opening is trying to achieve is far better placed than one who only remembers a list of moves.
The practical lesson is to learn the purpose behind the moves you know. If you understand why a pawn goes to a square and which pieces belong where, you can keep playing sensible moves after your memory runs out. Memorized lines without that understanding often fall apart the first time the opponent deviates.
A common mistake is to spend a long time on the first move after leaving known territory, then rush later. Another is to keep playing the move you expected as if the opponent had not deviated. When the opponent leaves the line, re-check what has changed: which tactics are new, which pawn breaks are available, and where each king is safe.
A neighboring idea is Book move. A book move is a move in a well established opening line, one that already appears in published analysis, opening databases or an engine's prepared opening list, so a player can choose it from knowledge rather than calculation.
It sits close to Early queen move. An early queen move is bringing the queen out in the first few moves, before the minor pieces are developed, which usually loses time because the opponent can attack it with developing moves.
Also worth reading is Opening explorer. An opening explorer is a tool that shows, for any position reached from the start, which moves players have chosen in past games and how often each move led to a win, draw or loss.
Opening ideas such as "Out of book" are easier to keep when they are tested on your own games, which is what a first lesson with George Li begins with.
Related terms
A book move is a move in a well established opening line, one that already appears in published analysis, opening databases or an engine's prepared opening list, so a player can choose it from knowledge rather than calculation.
Opening preparation is the work done before a game to know your chosen lines well, including studying your own repertoire and checking the likely choices of a specific opponent.
Opening theory is the body of knowledge about the opening, made up of the known moves, plans and evaluations of the main lines that players have tested over many games.
A novelty is a move that has not been played before in a known position of an opening, introduced to take the opponent out of their preparation and into unexplored territory.
Guides that go further
Most popular chess openings and win rates
The most played openings, how often White wins, and how the numbers change with rating, computed from public games.
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
A game review, a read on your level and a written study plan from George Li.
Want to spot this in your own games?
A first lesson starts with your games: George reviews them and builds a study plan around what he finds.