Chess Opening

In chess the word "opening" has two common meanings: a stage of a game and a sequence of moves. Chess players are so familiar with these two meanings that many chess writers never state the distinction and may switch without notice from one meaning to the other.^[1]

A chess opening is the group of initial moves of a chess game. Recognized sequences of opening moves are referred to as openings as initiated by White or defenses, as created in reply by Black. There are many dozens of different openings, and hundreds of named variants. The Oxford Companion to Chess lists 1,327 named openings and variants.^[2] These vary widely in character from quiet positional play (e.g. the Réti Opening and some lines of the Queen's Gambit Declined) to wild tactical play (e.g. the Latvian Gambit and Two Knights Defense, particularly the Traxler Variation).

A sequence of opening moves that is considered standard (often cataloged in a reference work such as the Encyclopedia of Chess Openings) is referred to as "the book moves", or simply "book". These reference works often present these 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.^[3] Professional chess players spend years studying openings, and continue doing so throughout their careers, as opening theory continues to evolve.

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.^[4]