This move is interesting. While some in the past considered it ridiculous, others point out that White wins a pawn in the process.

The White Knight now threatens a fork with Nxf7, yet to do so it's broken a rule of opening development in that the same piece moves twice; if Black plays correctly then White will seriously fall behind in development. Black's only sensible way to defend f7 is to block the bishop's access with 4...d5, but the pawn on d5 itself is not fully defensible by Black.

Black has two sensible replies.

• 4...d5 The best way for Black to defend is to play d5, blocking the bishop from the weak f7 square.
• 4...Bc5 Ignoring the threat, a risky option is the Traxler Attack. Black can try a bishop sacrifice on f2 if White plays Nxf7. Instead, White will often prefer Bxf7+.

Additionally, black may choose to move the knight on f6. This opens up an attack on the white g5 knight. There are two sensible methods of achieving this:

• 4...Nxe4.
• 4...Nd5.

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

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

Estimated next move popularity
d5 80%, Bc5 19%, Others less than 1%

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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.