White's choice is limited. The most common move is to retreat the knight by playing 4. Nf3.

The Karklins-Martinovsky Variation (4. Nd3!?) has been tried occasionally, and was notably played by Magnus Carlsen in the 2018 World Chess Championship against Fabiano Caruana.^[1]

The Paulsen Attack (4. Nc4) may look like another unique idea, but according to computers it is only as effective as, or even worse, than Nd3. It is been marked as ineffective by certain people,^{[citation needed]} but it is another idea that is worth trying.^{[citation needed]} On the positive side, no pawn is blocked, unlike in Nd3. The main issue with 4. Nc4 is that the knight is in the center of the board in an odd position and can't return to a natural development square, like f3 or c3. It will be chased around quite a bit.

White can also try the Cochrane Gambit, 4. Nxf7, a dangerous line for both players.

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

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6

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Wikipedia has related information at Petrov Defence

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