[<< wikibooks] Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...Bc5/5. d4 
== 5. d4 ==
Black's bishop on c5 is under attack. The most common reply is 5...d5, counterattacking White's bishop.

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