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...d6/8. d4

=Ruy Lopez Main Line=

8. d4?!
White counts the number of pieces attacking d4, and the number of pieces defending it, and reckons that 8. d4 is a safe move.

8. d4? Nxd4! 9. Nxd4 exd4 10. Qxd4
Alas, Black plays 10...c5! and wherever her beautifully centralised queen moves, Black follows up with 11...c4 and the b3-bishop is trapped. (third diagram)

The moral of the story: White must prepare d4 with 8. c3.