The opening is the initial stage of a chess game. It usually consists of established theory. The other phases are the middlegame and the endgame. Many opening sequences, known as openings, have standard names such as "Sicilian Defense". The Oxford Companion to Chess lists 1,327 named openings and variants, and there are many others with varying degrees of common usage.Opening moves that are considered standard are referred to as "book moves", or simply "book". When a game begins to deviate from known opening theory, the players are said to be "out of book". In some openings, book lines have been worked out for over 30 moves, such as some lines in the classical King's Indian Defense and in the Najdorf Variation of the Sicilian Defense.Professional chess players spend years studying openings, and they 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 less 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. Source: Wikipedia (en)