Four knights
1. e4 e5 2. Nf3 Nc6 3. Nc3 Nf6 4. Bb5 Bb4 5. O-O O-O 6. d3 Bxc3
White pushes the king's pawn to e4. Black plays the king's pawn to e5. White develops the knight to f3.
Black develops the knight to c6. White transfers the knight to c3. Black transfers the knight to f6.
White develops the light-squared bishop to b5. Black develops the dark-squared bishop to b4. White castles kingside.
Black castles kingside. White pushes the queen's pawn to d3. Black captures with the dark-squared bishop on c3.