Two Knights Defence

With 3...Nf6, Black develops a knight and attacks the e4-pawn, getting one step closer to castling. This move seems like the most obvious one Black can play in the Italian, but it also comes at a disadvantage of blocking the d8-h4 diagonal of the black queen.

White has several ways to proceed from this position: defend the e-pawn, usually with 4. d3, attack f7 with the aggressive 4. Ng5, or sacrifice the pawn for an open centre and swift attack.

The e4-pawn is attacked, so the most logical move for white would be to defend it.

4. d3 is the most common move, defending the pawn and opening the c1-h6 diagonal for the dark squared bishop. This is known as the modern bishop's opening. After 4...Bc5 this transposes into the Giuoco Pianissimo with. 4...Be7 continues with Ruy Lopez-style development with 4...Be7.

4. Nc3 transposes into the Italian variation of the four knights opening. Though this looks promising as it defends e4 while also developing a piece, it is less common and allows Black to play a centre fork trick. 4...Nxe4 5. Nxe4 d5 6. Bd3 dxe4 7. Bxe4 Bd6: Black temporarily sacrifices a piece in order to play d5 and get back the piece with a comfortable position and has equalised.

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 and Black equalises with best play.

The most serious continuation is 4...d5, sacrificing a pawn to relieve the pressure on f7, 5. exd5 Na5, the Polerio defence which counter-attacks the bishop.

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.

In a sense 4. Ng5 also defends e4, because 4...Nxe4? (called the Ponziani-Steinitz gambit) is answered by 5. Bxf7+! Ke7 6. d4.

4. d4, called the open variation, opens up the centre. This almost always transposes into the Scotch gambit, 4...exd4, and the main moves are 5. O-O or 5. e5. If 4...Nxe4? there is a trap, 5. dxe5 Bc4?? 6. Qd5!, threatening Qxf7# to pick up the knight.

White can also gambit the e4-pawn with 4. O-O?, intending to meet 4...Nxe4 with 5. Nc3 Nxc3 6. dxc3. White has sacrificed the pawn for a rapid lead in development and an open position, hoping for a swift attack on Black's uncastled king.

This line has historically also been known as the Prussian defence.^[1]

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

Wikipedia has related information at Two Knights Defence.

• The Two Knights Defence (1983) Yakov Estrin. ISBN 0-7134-3991-2.
• 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.