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

Black saves their knight and attacks the e5 pawn. White can choose to give the pawn back now or later in return for space and/or piece development. It is possible for White to hold onto the pawn, but at a positional cost. The main options are 4. e4, 4. Nf3, or 4. Bf4.

4. e4, the Alekhine variation gives the pawn back immediately in return for taking space in the centre. 4. e4 increases White's control over d5 and reveals an attack on Black's knight, so Black's move is almost always 4...Nxe4 5. f4 and Black chooses between 5...Nec6 or 5...Ng6. One idea for Black is ...Bb4+, hoping for Nc3 Bxc3+ bxc3 leaving White with doubled isolated c-pawns.

4. Nf3, the Adler variation, develops a piece while defending the pawn. Black can force White to give back the pawn in this line because of the move 4...Bc4: this compels White to play 5. e3 to end the pressure on f2, so White cannot then play Bf4 and so cannot bring enough defenders to the pawn. (Anything other than 5. e3 is a horror show, e.g. 5. Bf4?? Bxf2+ 6. Kd2 d6 7. exd6 Qf6 8. e3 Nxe3 9. Bxe3 Qxb2++-).

After 5. e3, White can concentrate on development and let Black pick up the pawn. A continuation is 5...Nc6 6. Nc3 O-O 7. Be2 Re8 8. O-O Ngxe5 9. Nxe5 Nxe5. A common middlegame plan for Black is to rook lift with ...a5, ...Ra6, and ...Rh6.

Alternatively 4...Nc6 allows White to bring in the bishop to defend the pawn, 5. Bf4, and transposes into a Rubinstein variation.

4. Bf4 is the Rubinstein variation and mainline continuation. Here 4...Bc4? is a mistake because 5. e3 obstructs f2 and discovers an attack on Black's knight: 5...d6 (to allow the queen's bishop to defend the knight) 6. exd6 cxd6 and White gets to stay a pawn up.

The continuation then is 4...Nc6 5. Nc3 Bb4+ and White then has to decide how badly they want to keep the pawn (see diagram). Black has two attackers on e5 and can bring a third with ...Qe7. White has two defenders, and to bring a third will need to play Qd5:

• Blocking the check with 6. Nc3 keeps the option of Qd5 open but allows 6...Bxc3 bxc3 and White's doubled, isolated c-pawns will be a long-term weakness.
• The mainline is 6. Nd2, blocking the check and preventing Qd5 so allowing Black to win back the pawn after 6...Qe7.

But either approach is playable for White. A famous trap is 6. Nd2 Qe7 7. a3 (attacking the bishop?) Ngxe5 8. axb4?? Nd3# (the e-pawn is pinned by Black's queen).

4. Qd5?!, to defend the pawn and attack Black's knight, is a mistake. 4...d6 attacks the pawn and lets the queen's bishop defend the knight, and ...Nc6 to kick the queen is on the way. 5. exd6 (to "cash in" the pawn) let's Black's bishop develop 5...Bxd6 and White must be careful not to blunder ...Bb4+ discovered attack on White's queen.

With 4. f4? White is determined to hold onto the pawn, but it comes at a high cost. After 4...Bc4 White must avert the threat of Bf2+ or Nf2 with a fork and easily falls behind, e.g. 5. Nh3 (defends f2) O-O 6. e4 (readies bishop to develop, and discovered attack on Black's knight) d6 (defends knight) 7. exd6 Re8∓ threatens ...Rxe4+ Be2 Qh4+.

For explanation of theory tables, see theory table and for notation, see algebraic notation.

1.d4 Nf6 2.c4 e5 3.dxe5 Ng4

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Wikipedia has related information at Budapest Gambit