After 1. P to K 4, P to K 4; 2. Kt to K B 3, Black may defend the attacked King's pawn with 2.... P to Q 3, and this constitutes Philidor's Defence.

WHITE.BLACK.
1.P to K 41.P to K 4
2.Kt to K B 32.P to Q 3
3.P to Q 43.P takes P
4.Kt takes P...

If 4. Q takes P, Black replies 4.... Kt to Q B 3 (best); 5. B to Q Kt 5, B to Q 2; 6. B takes Kt, B takes B; 7. B to Kt 5, Kt to B 3; 8. B takes Kt.

Q takes B; 9. Q takes Q, P takes Q; 10. Kt to B 3, and Black has two Bishops against two Knights, and an open Kt and K file for his Rooks, and consequently the advantage.

...4.Kt to K B 3
5.B to Q B 45.B to K 2
Equal game.
Second Variation.
1.
P to K 4
P to K 4
2.
Kt to K B 3
P to Q 3
3.
B to B 4
P to K B 4[[94]]
4.
P to Q 4
K P takes P
5.
Kt to Kt 5
Kt to K R 3
6.
Kt takes P
Q to K 2[[95]]
7.
Q to R 5: ch.
K to Q 2
8.
B to Kt 5
Q takes P: ch.
9.
K to Q 2
Q to Kt 5
10.
Q to Kt 6
R takes Kt
11.
B to Q 5
P to B 3[[96]]
12.
P to K R 3
Winning the Q.
Third Variation.
Repeat the opening of second variation up to Black's fourth move.