411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Bc5/4. c3/4...Nf6
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Central variation Central variation C64
4...Nf6
4...Nf6 is the most common response to the Central variation.
This invites 5. d4, attacking the bishop. Black can take the pawn, 5...exd4, and after 6. cxd4 Black can get their bishop out of trouble with check and then take the e4 pawn (e.g. 6...Bb4+ 7. Nc3 Nxe4). (6...Bb3+ . However, White has the intermezzo 6. e5!, protecting the e4 pawn and kicking Black's knight.
Alternatively, 5. O-O transposes into the Zukertort gambit. Black can take the e4 pawn but White can recover it after Qe2.
Theory table
References
See also
Ruy Lopez