Philidor - exchange variation
1. e4 e5 2. Nf3 d6 3. d4 exd4 4. Nxd4 Nf6
White plays the king's pawn to e4. Black plays the king's pawn to e5. White develops the knight to f3.
Black plays the queen's pawn to d6. White plays the queen's pawn to d4. Black captures on d4 with the king's pawn.
White captures with the knight on d4. Black develops the knight to f6.