In chess, "theory" refers to the large and ever-expanding body of prevailing wisdom and assumptions about how the game chess should be played in certain positions. Chess theory can be studied by reading a vast body of written chess literature dating back hundreds of years, by studying online databases of millions of recorded chess games, or by using a computer chess engine to learn which moves might be best. Brand new chess theory can be created by analyzing new lines in openings or discovering new principles of play in the middlegame and endgame. Chess experts who develop new chess theory and publicize it to the world in their writings are known as "chess theoreticians."

Chess theory has become so well developed in certain opening lines, that at the highest levels of chess today, grandmasters often play as many as 15 or more moves out of the opening that have already been played many times before and are well understood by chess theory. Such games are said to be still "in theory" or "in book" until the first move is finally made that has not been seen before or written about in chess theory. Such moves are known as "novelties," and are contrasted to "known moves" that have already been covered by chess theory. Once a novelty has finally been played, the game is said to be "out of theory" or "out of book."

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