Chess Opening Theory/1. e4/1...d5/2. exd5/2...Nf6/3. d4/3...Bg4

Black develops their bishop with tempo on White's queen. White will have the time to secure their pawn on d5 and so hold on to the extra material, but Black hopes instead to gain the advantage in piece development and activity.

White's first priority must be to parry the threat of ...Bxd1. There are several ideas: 4. Be2, 4. Ne2 or 4. Nf3, and 4. f3. Additionally, White may play the intermezzo 4. Bb5+.

4. f3 is the main move and perhaps the most natural appearing, as it both prevents ...Bxd1 and attacks the bishop. The trade-off that it takes away the f3 square from White's knight is small. Black can afford to spend the tempo moving the bishop twice because they already have two pieces developed, and all White has moved are some pawns. If White is not careful and allows the centre to open before they have castled, they can easily succumb to Black's active play (see diagram).

After 4...Bf5, White usually moves to secure the d5 pawn, either 5. c4 (the Banker variation) or 5. Bb5+ Nbd7 6. c4 but the most testing move (called the correspondence refutation) is 5. g4! (kicking the bishop again) Bg6 first, then 6. c4, and it is hard for Black to make anything of their development lead. White has the threat of g5, and with the c4 and f3 pawns controlling the more active squares, Black's knight is forced to a passive position like ...Nfd7 or ...Nh5.

4. Nf3 interposes the knight instead. This avoids weakening the king's side pawn structure and develops a piece, getting White closer to castling. This pins White's knight, but they can soon play Be2 to defuse the pin and castle. 4. Nf3 offers the d5 pawn back to Black, 4...Nxd5 (the Gipslis variation) or 4...Qxd5 (4...Bxf3 5. Qxf3 Qxd5 6. Qxd5 Nxd5 is also reasonable).

4. Be2, the Wuss^[1] variation, allows Black to trade bishops then recover the pawn. 4...Bxe2 5. Nxe2 Qxd5 6. O-O=.

4. Bb5+ is a challenging intermezzo because White's d5 pawn gives them more control over the c6 square. If 4...c6 (the elbow variation) 5. dxc6 Bxd1?? is an error. 6. c7+ (discovered check) Nbd7 (if 6...Qd7?? 7. c8=Q#) 7. cxd8=Q+ Rxd8 8. Kxd1+- and White has recovered the material with considerable interest. Though 4...c6 5. dxc6 Nxc6 6. Nf3 may be played, 4...Nbd7 to block the check is more common and Black can think about kicking the bishop with ...a6, or ...c6 after they have dealt with the pawn on e6. 5. f3 Bf3 6. c4 is called the Jaboul variation.

The line had some popularity in the late 1990s. Roush, Lance and Cornell's monograph on the gambit attributes the christening of the line to John Donaldson in 1996, who recognised the contribution of a number of Portuguese players.^[2]

Portuguese players who were exploring the line at the time included Carlos Pereira dos Santos^[3] and Sergio Rocha.^[4]

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

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