1.d4 Nf6 2.c4 e6 3.Nc3 Bb4
With 3...Bb4, Black prevents white from playing an immediate 4.e4 (4...Nxe4) and Black threatens to play Bxc3 doubling the white pawns. White try the Samisch (4.a3) which forces the immediate exchange on c3. The Leningrad with 4.Bg5 h6 5.Bh4 c5 leads to very sharp play with chances for both sides.
Garry Kasparov prefers to play 4.Nf3 and 5.g3. The Rubinstein variation (4.e3 and Nge2) is very solid with the aim of supporting the Nc3. Currently, white's most often played line in the Classical variation with 4.Qc2. With this line, white avoids the doubling of the c-pawns.