The chief sideline to 3. Nxe5, with 3. d4 White seeks to open the centre immediately and offers Black their choice of pawns to capture.

3...exd4!? is playable but less conventional as it allows 4. e5! to kick the knight. Typically this leads to trading both d- and e-pawns and opening the centre fully: the main continuation is 4...Ne4 5. Qxd4 (the Centre attack) d5 (defends knight) 6. exd6 e.p. Nxd6 7. Nc3, although there are some sidelines (5. Qe2 was advocated by Steinitz; 5. Bb4+ is the Tal gambit).

3...Nxe4 is the main line and leads to a trade of e-pawns. White could retake immediately (4. Nxe5 or 4. exd4) but the normal move order is 4. Bd3 d5 5. Nxe5 where induces ...d5 first, so Black doesn't have ...d6 to shoo the knight away.

3...d5, the Symmetrical variation, passes the decision on what to take first back to White. Since Black has played ...d5 of their own volition, White may choose 4. Nxe5 and the positions can transpose after 4...Nxe4 5. Bd3 (4...dxe4? 5. Bc4 [threatening Nxf7] Be6 6. Bxe6 fxe6 and Black has isolated double e-pawns±).

3...d6 transposes to the Nimzowitsch variation of the Philidor defence.

Jaenisch credited Petrov with 3. d4,^[1] who had perhaps the earliest games with it.^[2] A later advocate of the line was Steinitz, who wrote in his 1889 book:^[3]

As regards its [2...Nf6] merits as a defence various authorities have expressed different opinions on the subject, but all agreed hitherto that 3 Kt×P [3. Nxe5] was White's best continuation though it was generally admitted that this line of attack only retained the advantage of the first move by proper play on the other side. We think however that the superior position at least can be proved for the first player by the attack 3 P—Q4 [3. d4] which has hitherto been almost ignored.

Because of his endorsement, some opening books (e.g. Chess.com's opening explorer) give 3. d4 the appellation Steinitz attack instead. Others (e.g. Lichess.org's) reserve it for the continuation 3...exd4 4. e5 Ne4 5. Qe2, which Steinitz preferred to 5. Qxd4.

3. d4 particularly came into vogue in the 1990s, where it was adopted at the highest levels including by Kasparov^[4] and Ivanchuk.^[5]

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

1. e4 e5 2. Nf3 Nf6 3. d4

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Wikipedia has related information at Petrov's Defence