411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nc3/3...Bb4/4. Bc4
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Three Knights Game
4. Bc4
Black has a few options here:
/4... O-O/
/4... d6/
/4... Bxc3/