Chess opening

The opening is the initial stage of a chess game. The other phases are the middlegame and the endgame. A large amount of established opening theory exists, with some theory dating back centuries. Many opening sequences, known as openings, have standard names, such as 1.e4 c5, which is named the "Sicilian Defense"; these can be broken down further into variations. The Oxford Companion to Chess lists 1,327 named openings and variations, 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 theory, the players are said to be "out of book". In some openings, book lines have been worked out for over 30 moves. Standard openings have a variety of aims, including developing rapidly, harmoniously, and actively, controlling the center, and king safety. There are also psychological goals and other gameplay-related goals.

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.