Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ng4/4. Bf4

This is called the "Rubinstein variation" in reference to the famous game Rubinstein – Vidmar (Berlin 1918) where the move 4.Bf4 was first employed. The move 4.Bf4 first aims to be able to answer 4...Bc5 with 5.e3 without blocking the Bc1, contrary to what happens in the Adler line 4.Nf3. On the other hand, the early development of the bishop means that White is slightly exposed to a Bb4+.

Also, in the Adler line White faces the risk of a strong attack against his kingside, while in the 4.Bf4 variation this is seldom the case because White's Bf4 is well placed to protect White's kingside if needed. However, in some cases the Bf4 can become slightly exposed (e.g. some variations of the Rubinstein line when Black plays g7–g5 and h7–h5).

Apart from the sideline 4...g5, the main line continues with both camps developing their pieces around the e5-pawn with 4...Nc6 5.Nf3 Bb4+ (preparing 6...Qe7) when White has an important choice to make between the two moves 6.Nc3 and 6.Nbd2, each leading to extremely different play. With 6.Nc3 White accepts to have his pawn structure on the queenside ruined, in return for a material advantage of one pawn, the bishop pair and active play in the center.

On the other hand, with 6.Nbd2 White gives back the gambited pawn in order to keep a healthy pawn structure and to get the bishop pair. After 6...Qe7 White generally plays 7.a3 in order to force the immediate exchange of the Bb4 for the Nd2 and get the bishop pair, a space advantage and a minority attack on the queenside. White can also try 7.e3 which tries to win a tempo over the 7.a3 variation but may end up with the Bb4/Nd2 exchange made in less favorable circumstances, or not made at all. The maverick gambit 6...f6 or 6...d6 also exist.

Theory table
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Bf4