Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ng4/4. Nf3/4...Nc6

The 4...Nc6 line
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Nc6

Black plays this when he wants to postpone the placement of its dark-squared bishop. Now White has a wide choice:
 * 5.Bf4 transposes in the 4.Bf4 variation explained hereafter.
 * 5.Qd5 transposes in the minor line 4.Qd5 explained hereafter.
 * 5.Nc3 will transpose into the 4...Bc5 line if Black plays 5...Bc5, but Black can also wait a bit to see what White is up to, e.g. 5...Ngxe5 6.Nxe5 Nxe5 7.Qc2 Bb4 when both players are still hesitating to castle long or short.

5.e3 Bb4+
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Nc6 5.e3 Bb4+

After 5.e3 the check 5...Bb4+ is not that good because White can react with the simple 6.Bd2 that does not concede anything and keeps the possibility for his Nb1 to reach the important d5-square. A game continued 6...Bxd2+ 7.Qxd2 O-O 8.Be2 Ncxe5 9.Nxe5 Nxe5 10.O-O d6 11.Nc3 Bg4 12.f3 Be6 13.b3 Qh4 and here Borik advises 14.Nd5 with the more comfortable game.

The other reply 6.Nc3 is good only if Black makes the errors of not doubling White's pawns with the immediate 6...Bxc3+. For example in a game after 6...Ngxe5 7.Bd2 O-O 8.a3! Bxc3 9.Bxc3 d6 Black had no compensation for the loss of the bishop pair. After the correct 6...Bxc3+ 7.bxc3 Qe7! (important to prevent both c4–c5 and Bc1–a3) 8.a4 Ngxe5 9.Ba3 d6 10.c5 Nxf3+ 11.gxf3 Qe5! 12.Qd2 dxc5 and White had not sufficient compensation for the pawn.

5.e3 Ngxe5
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Nc6 5.e3 Ngxe5

Better for Black is 5...Ngxe5 when Black can go into a kind of King's Indian Defence setup with g7–g6 and Bf8–g7. Then the pressure along the a1–h8 diagonal can be enhanced via the quick advance a7–a5–a4–a3. For example after 5.e3 Ngxe5 6.Be2 g6 ("!?" Lalic) 7.O-O Bg7 8.Nc3 O-O 9.Qd2 d6 10.h3 ("?" Lalic) 10...a5 ("!" Lalic) 11.b3 a4 and now 12.Bb2 would have been followed by 12.a3! 13.Bc1 Nxf3+ 14.Bxf3 Qf6 winning the Nc3.

5.Bg5
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Nc6 5.Bg5

5.Bg5 Be7 6.Bxe7 (6.Bf4 Bb4+ transposes in the 4.Bf4 variation) 6...Qxe7 7.Nc3 with the dangerous positional threat Nc3–d5. Here Borik advocates 7...Qc5 8.e3 Ngxe5, when he can react to Qd1–d5 with Qc5–e7 (and the d5-square is no more available to the Nc3), and to Nc3–d5 with Nc6–e7 (to exchange the annoying knight). Black can also delay the recapture of the e5-pawn with 7...O-O 8.Nd5 Qd8 9.e3 Ngxe5 10.Nxe5 Nxe5. Meanwhile, the natural 7...Ngxe5 falls into White's positional trap and after 8.Nd5 ("!?" Lalic) 8...Qd8 9.Nxe5 Nxe5 10.Qd4 f6 11.f4 Ng6 12.Qe4+ Kf7 White got an edge.

Theory table
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Nf3 Nc6