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The London System is an opening system in chess that can be used against virtually any black defense and thus comprises a smaller body of opening theory than many other openings. Also known as the "Mason Variation," it is a line in the Queen's Pawn Game where White opens with 1. d4, but does not play the Queen's Gambit, instead opting to rapidly develop the dark squared bishop. This will often result in a closed game. Because of its solid reputation, the London System has faced criticism for its lack of dynamic play.
The rapid development of the dark squared bishop in the London System may be contrasted to the Colle System - another popular Queen's Pawn Game - in which the queen's bishop often remains on c1 during the opening phase of the game.