1.d4 Nf6 2.c4 e6 3.Nc3 Bb4
Named after Aron Nimzovich, Black's last move stops White from playing e4. It also threatens to ruin White's pawn structure by exchanging the Bishop on b4 for the Knight on c3. This will weaken the Pawn on c4 which can no longer be protected by another Pawn. White has many possible responses, including ignoring the threat.
(ECO: E20)
