Key ideas
- The cost is time: the queen moves early, the kingside is undeveloped, and the queen no longer guards d4. Black uses those tempi to strike with ...d5, ...c5, or ...Nc6 followed by ...d6 and ...e5.
- 4...O-O 5.a3 Bxc3+ 6.Qxc3 is a main line. Black can continue 6...b6 (the Keres Defense) to contest the long diagonal, or play for ...d5 and ...c5.
- 4...d5 is the Noa Variation. After 5.a3 Bxc3+ 6.Qxc3 Ne4 7.Qc2 c5, Black uses the knight on e4 and the ...c5 break to gain time against White's queen.
- 4...c5 (the Berlin Variation) invites 5.dxc5, after which Black usually castles and regains the pawn with ...Bxc5. 4...Nc6 (the Zurich Variation) aims for ...d6 and ...e5, using the fact that d4 has lost the queen's protection.
Plans for White
Play a3 to gain the bishop pair without doubled pawns, develop with Nf3, Bg5 or e3, and use the two bishops and the dark squares. Watch the king: with the queen on c3 or c2 and the kingside undeveloped, an early opening of the position can be dangerous.
Plans for Black
Hit back in the centre while White is behind in development, with ...d5 or ...c5, and use ...Ne4 to harass the queen. If White gets the bishop pair, keep the position closed enough for the knights, or exchange a bishop with ...b6 and ...Bb7 or ...Ba6.
For the wider picture, see the Nimzo-Indian Defense guide.