Nimzo-Indian Defense: Three Knights Variation, Korchnoi Variation

The Nimzo-Indian Defense: Three Knights Variation, Korchnoi Variation is a chess opening classified under ECO code E21 in the Indian Defences family, where Black responds to 1.d4 with 1...Nf6, including the Nimzo-Indian, Queen's Indian, and King's Indian. It is reached after 5 moves: 1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Nf3 c5 5. d5. The resulting position leaves black to move.