Queen's Gambit Declined: Been-Koomen Variation

The Queen's Gambit Declined: Been-Koomen Variation is a chess opening classified under ECO code D50 in the Closed & Semi-Closed Games family, arising from 1.d4 d5, including the Queen's Gambit, Slav Defense, and related systems. It is reached after 4 moves: 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 c5. The resulting position leaves white to move.