Ruy Lopez - Berlin defence - 4.O-O
1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. O-O
White plays the king's pawn to e4. Black moves the king's pawn to e5. White develops the knight to f3.
Black develops the knight to c6. White develops the light-squared bishop to b5. Black jumps the knight to f6.
White castles kingside.