Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e5/3. dxe5/3...Ng4/4. Bf4/4...Nc6/5. Nf3/5...Bb4/6. Nc3

This line is called the "Kornl Richter gambit" by the chess historian Bill Wall.; this is the only line of the Budapest gambit in which Black cannot regain his pawn for sure. Black does best to immediately exchange the Nc3 with 6...Bxc3+ 7.bxc3 as otherwise White gets a small positional advantage simply by avoiding the doubled pawns. Then Black can put pressure on the e5-pawn with 7...Qe7 when White's only possibility to keep the pawn is 8.Qd5. At this stage, White's position has several defects notably: White also has long-term trumps, however, and notably he is still a pawn up (the e5-pawn) and he has the bishop pair. Moreover, White currently threatens to ease the pressure with the move 9.h3 that would put the Ng4 on the unfavourable square h6, so Black must keep the initiative with active moves like 8...Qa3 (attacking the c3-pawn) or 8...f6 (attacking the e5-pawn once more).
 * two pairs of doubled pawns (in c3&c4, and in e2&e5),
 * a king in the centre who still needs two tempi before being able to castle,
 * a queen in the centre as well, who will be subject to attacks by black minor pieces.

Line 8...Qa3
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Bf4 Nc6 5.Nf3 Bb4+ 6.Nc3 Bxc3+ 7.bxc3 Qe7 8.Qd5 Qa3

The line 8...Qa3 puts pressure on the white queenside pawns, this pressure can latter be reinforced with the manoeuvre Nf6–e4. The black queen can also access to the a5-square at some point, where it puts pressure on the e1–a5 diagonal towards the white king.

An interesting sideline is 9.Rd1!? aiming to open the position and seize the initiative even at the cost of the doubled pawns. For instance, after 9...Qxc3+ 10.Qd2!? Qxc4 11. h3 Nh6 12.e3 Qb4 13.Qxb4 Nxb4 14.a3 Nc6 15.Bc4 White has good compensation for the pawn thanks to its bishop pair and good development. Another fringe possibility is 9.Qd3, after which Mieses continued by 9...Qa5 in his game against Rubinstein. The game followed up with 10.Rc1 Ngxe5 11.Nxe5 Nxe5 12.Qg3 with a double attack against the pawns in c7 and g7. White will be a pawn up but his queenside pawn are weak and he is slightly behind in development.

The most common move, however, is 9.Rc1 playing on the fact that 9...Qxa2?! 10.h3 Nh6 11.e4 Qa3 12.Be2 gives White a clear advantage with the bishop pair and the black Nh6 awkwardly placed. So Black prefers 9...f6 10.exf6 Nxf6 and White has to choose between three possible queen retreats:
 * The simplest is 11.Qd1!? d6 12.Qb3 and White has a slight advantage. He retains the extra pawn, and the black queen would be better of on the e7-square than on the a3-square.
 * Strangely enough, 11.Qd2 is the most frequently played although it is not the best: Lalic thinks 11.Qd2 is inappropriate and gives Black excellent counterplay, while Moskalenko considers that the white queen is passive on the d2-square. After 11.Qd2 d6 12.Nd4 0-0 we reach the position of a famous game between Rubinstein and Vidmar, the first tournament game between grandmasters with the Budapest gambit. Rubinstein erred with 13.e3? when the Bf4 is cut from the queenside, so that Black can show the weakness of White's dark squares with 13...Nxd4! 14.bxc3 Ne4 15.Qc2 Qa5+ 16.Ke2 Rxf4+!. The better 13.f3 occurred in a game between O'Kelly and Heidenfeld. The correct method for Black is to target the c4-pawn with the regrouping Ne5/Qc5. After 13...Ne5 14.e4 Qc5 15.Nb3 Qc6 16.Bxe5!? dxe5 17.Qg5 Re8 18.Be2 and here Moskalenko proposes either 18...a5 to attack the queenside at once and capture the a2-pawn, either 18...b6 to block the c4-pawn and attack it later.
 * After 11.Qd3 0-0 12.g3 d6 13.Bg2 Black should switch to a materialistic mode with 13...Qxa2.

Line 8...f6
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Bf4 Nc6 5.Nf3 Bb4+ 6.Nc3 Bxc3+ 7.bxc3 Qe7 8.Qd5 f6

In the line 8...f6 Black concentrates on an active piece play in the centre. After 9.exf6 Nxf6 all three queen retreats 10.Qd1, 10.Qd2 and 10.Qd3 are possible but each of them has its own drawbacks: on d1 the queen is not developed, on d3 it is exposed to the manoeuvre Bc8–f5 and on d2 it is exposed to the manoeuvre Nf6–e4. Lalic considers 10.Qd3 to be the main move (as Moskalenko also does), qualifies 10.Qd1 as a "respectable option", but considers 10.Qd2 as "inaccurate", giving the line 10...d6 11.e3 Ne4 12.Qc2 g5 13.Bg3 Bf5 14.Bd3 h5 with the initiative for Black. Meanwhile Black will try to create counterplay by attacking either the weak c4-pawn, or more seldom the kingside with the pawn thrusts g7–g5 and h7–h5. In both cases a key possibility is the move Nf6–e4 that centralises the black knight, attacks the weak c3-pawn, controls the c5-square and supports the g7–g5 pawn advance.

Line 8...f6 followed by 11.e3
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Bf4 Nc6 5.Nf3 Bb4+ 6.Nc3 Bxc3+ 7.bxc3 Qe7 8.Qd5 f6 9.exf6 Nxf6 10.Qd3 d6 11.e3

Meanwhile Black will try to create counterplay by attacking either the weak c4-pawn, or more seldom the kingside with the pawn thrusts g7–g5 and h7–h5. In both cases a key possibility is the move Nf6–e4 that centralises the black knight, attacks the weak c3-pawn, controls the c5-square and supports the g7–g5 pawn advance.
 * The choice of 10.Qd1 is not very active, and White will lose a tempo when it develops his queen again. The game may continue with 10...d6 11.e3 0-0 12.Be2 Ne4 13.Rc1 Kh8 and White does better to avoid 14.0-0?!, when 14...g5 launches a powerful attack.
 * The first game to stage this variation with 10.Qd3 was between Rubinstein and Schlechter. After 10...d6 11.e3 0-0 12.Be2 Ne4 13.Qc2 Nc5 14.Nd4 Ne5 15.0-0 Bd7 Black holds on well, and here Moskalenko recommends 16.Qe2 Rae8 with complicated play. Instead of 15...Bd7 Black can also try the direct 15...Qf7 to press the c4-pawn. Another idea for Black is the push 12...b6, developping the light-squared bishop on the a8–h1 diagonal instead of the more usual developments on d7, f5 or g4. The game continued 13.0-0 Kh8 14.Qc2 Bb7 15.Nd4 Ne5 16.Rae1 Rae8 17.Bg5 Qf7 18.Bxf6 gxf6! and Black used the g-file and his Bb7 to attack White's king.

Line 8...f6 followed by 11.g3
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Bf4 Nc6 5.Nf3 Bb4+ 6.Nc3 Bxc3+ 7.bxc3 Qe7 8.Qd5 f6 9.exf6 Nxf6 10.Qd3 d6 11.g3

As the development 11.e3 12.Be2 leaves his light-squared bishop a bit passive, White can improve its prospects with 11.g3 0-0 12.Bg2 and a possible attack along the a8–h1 diagonal. For example after 12...Ne4 13.0-0 Nc5 14.Qe3 Be6 the freeing move 15.Nd4! allows White to seize the initiative. That is why Black should avoid the natural manoeuvre Nf6–e4–c5 and prefer 12...Bg4! instead, to immediately puts pressure on the e2-pawn. After 13.0-0 Rae8 14.Rfe1 Kh8! the struggle continues. White has also tried 10.Qd3 d6 11.Bg5 to give up the bishop pair in return for better control on the centre and the e4-square, and after 11...0-0 12.e3 Lalic recommends 12...b6 to occupy the a8–h1 diagonal.

This line is actually much stronger than the line 11.e3, as the light-squared bishop will have much greater prospects on the a8-h1 diagonal than on the a6-f1 diagonal. One of its strength is that White can combine the influence of the Bg2 with the influence of a rook on the b-file, so as to create strong pressure on the black queenside.

Theory table
1.d4 Nf6 2.c4 e5 3.dxe5 Ng4 4.Bf4 Nc6 5.Nf3 Bb4+ 6.Nc3