The Colle System, also known as the Colle–Koltanowski System, is a chess opening system for White, popularised in the 1920s by the Belgian master Edgard Colle and further developed by George Koltanowski.
The Colle is characterised by several moves. White's center pawns are developed to d4 and e3, the king's knight is developed to f3, the king's bishop is developed to d3, and the queen bishop's pawn (c-pawn) is developed to c3. Common continuations include development of the queen's knight to d2 (Nbd2) and kingside castling (0-0). A major theme of the Colle System is the ambition to play a well-timed e4, where the square is defended by the bishop on d3, the knight on d2 (following Nbd2), and possibly the rook on e1 (following 0-0 and Re1). Although sometimes described as a specific sequence of moves, the Colle System is not a fixed line of play, but rather a system for White where the moves may be permuted at the player's discretion. When a game opens with most or all of the above moves, to the exclusion of moves which typify other openings, the game may be described as "a Colle System".
Although criticised by some and used infrequently in master play, the Colle is regarded by others as a solid and playable opening.