Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5

4. Ng5!? is a sideline but the most common amateur move. This Romantic move is an attack on f7 with the bishop and knight, taking advantage of the fact that Black gave up control of g5 and cannot castle yet. However, it is unprincipled to attack before having finished development―Siegbert Tarrasch slammed the move as "bungling"^[1]―and Black equalises with best play.

Black needs to defend f7: otherwise, as in the common beginner blunder, 4...h6?? 5. Nxf7 Qe7 6. Nxh8, Black gets the house. If Black thinks to counter-attack instead, 4...Nxe4?, the Ponziani-Steinitz gambit, fails to 5. Bxf7+! Ke7 6. d4.

4...d5 is the best way to defend f7. This sacrifices a pawn to 5. exd5. Then there have been several moves tried:

• 5...Na5, the Polerio defence, is the most serious continuation and main line today. Black counterattacks the bishop, and the line continues 6. Bb5+ c6 7. dxc6 bxc6 8. Bd3 Nd5 9. Nf3 Bd6 10. O-O. White may be ahead a pawn, but Black is considered equal with more active pieces;
• 5...b5!?, the Ulvestad variation, to drive the bishop on c4 to somewhere less useful;
• 5...Nd4!?, the Fritz variation, gets the knight out from under the gun;
• 5...Nxd5?! is more common in amateur games, allowing 6. Nxf7, the audacious knight sacrifice known as the fried liver attack, or 6. d4, the Lolli attack.

4...Bc5? is the venomous but objectively dubious Traxler counterattack. Black can answer 5. Nxf7? Bxf2+! 6. Kxf2 Nxe4+=, but instead 5. Bxf7+ is a free pawn.

1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5

Wikipedia has related information at Two Knights Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

• Nunn's Chess Openings. 1999. John Nunn (Editor), Graham Burgess, John Emms, Joe Gallagher. ISBN 1-8574-4221-0.