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

= Ruy Lopez:Open Game = 4.O-O is the most common reply to the Berlin Defence. This move protects the king by moving it out of the center. It also brings the rook closer to the center. If 4...Nxe4, then 5.Re1, and when the knight retreats, e5 falls after 6.Nxe5. If Black plays 4...Be7 (a rare reply), White should also play 5.Re1, preventing the threat of 5...Nxe4 6.Re1 Nd6; if 7.Nxe5, there is 7... Nxb5.

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