Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O/5...Be7/6. Re1/6...b5/7. Bb3/7...O-O/8. c3/8...d5/9. exd5/9...Nxd5/10. Nxe5/10...Nxe5/11. Rxe5/11...c6/12. d4

12...Bd6 is the universal response to 12. d4. Black's e7-bishop is hitting thin air on g5 and h4, so it relocates with gain of tempo to hit the much more exciting h2 square.