Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nf6/4. O-O/4...Nxe4/5. d4

= Ruy Lopez:Open Game = Here Black has taken a pawn, but although both black and white have 2 developed pieces, the white rook on f1 and white's queen have more mobility, and white is already castled. Because of white's slight lead in development he is playing d4 to further open the position and make better use of his better development

Theory table
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 * 1.e4 e5 2.Nf3 Nc6 3.Bb5 Nf6 4.O-O Nxe4 5.d4