Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...d5/5. exd5/5...Nd4/6. c3/6...b5/7. Bf1/7...Nxd5/8. Ne4/8...Qh4/9. Ng3/9...Bg4/10. f3

=Two Knights Defence:Fritz variation=

9. f3
Forced, no undefended squares for the White Queen to go to and 9. Be2 would lose a piece to 9...Nxe2 with threat of Nf4 and Ne2 or Nd3 mate.

Theory table
.
 * 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 b5 6.Bf1! Nd4 7.c3 Nxd5 8.Ne4 Qh4 9. Ng3 Bg4