opening

A chess opening or simply an opening refers to the initial moves of a chess game. The term can refer to the initial moves by either side, White or Black, but an opening by Black may also be known as a defense. There are dozens of different openings, and hundreds of variants. The Oxford Companion to Chess lists 1,327 named openings and variants. These vary widely in character from quiet positional play to wild tactical play. In addition to referring to specific move sequences, the opening is the first phase of a chess game, the other phases being the middlegame and the endgame.
Opening moves that are considered standard (often catalogued in a reference work such as the Encyclopaedia of Chess Openings) are referred to as "book moves", or simply "book". Reference works often present move sequences in simple algebraic notation, opening trees, or theory tables. When a game begins to deviate from known opening theory, the players are said to be "out of book". In some opening lines, the moves considered best for both sides have been worked out for twenty to twenty-five moves or more. Some analysis goes to thirty or thirty-five moves, as in the classical King's Indian Defense and in the Sveshnikov and Najdorf variations of the Sicilian Defense. Professional chess players spend years studying openings, and continue doing so throughout their careers, as opening theory continues to evolve. Players at the club level also study openings but the importance of the opening phase is smaller there since games are rarely decided in the opening. The study of openings can become unbalanced if it is to the exclusion of tactical training and middlegame and endgame strategy.A new sequence of moves in the opening is referred to as a theoretical novelty. When kept secret until used in a competitive game it is often known as a prepared variation, a powerful weapon in top-class competition.

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