Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nd4

Ruy Lopez - Bird's Defence
Black gives up the defense of e5 and instead counter-attacks with a threat towards the bishop. The drawback is that after the knight exchange on d4 Black will have a slightly inferior pawn structure, though with a slight space gain.

Theory table
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 * 1.e4 e5 2.Nf3 Nc6 3.Bb5 Nd4