4.
...
B P takes P
5.
Kt takes P
P takes Kt
6.
Q to R 5: ch.
K to Q 2
7.
Q to B 5: ch.
K to B 3
8.
Q takes P (K 4)[[97]]
P to Q R 3
9.
P to Q 5: ch.
K to Kt 3
10.
B to K 3: ch.
B to B 4[[98]]
11.
B takes B: ch.
K takes B
12.
P to Kt 4: ch.
K takes P [[99]]
13.
Kt to Q 2
Q to B 3
14.
R to Kt sq.: ch.
K to R 5
15.
P to Q B 3
P to Q R 4[[100]]
16.
B to Kt 5: ch.
K to R 6
17.
Kt to B 4: ch.
K takes P
18.
R to Kt 2: ch.
K to R 8
19.
Castles: checkm.

This variation has been selected to illustrate mate being given by castling.

THREE KNIGHTS' GAME.

The Three Knights' Game is brought about if, after 1. P to K 4, P to K 4; 2. Kt to K B 3, Kt to Q B 3, White plays 3. Kt to B 3, The following is an example:

WHITE.BLACK.
1.P to K 41.P to K 4
2.Kt to K B 32.Kt to Q B 3
3.Kt to B 33.B to Kt 5
4.B to Kt 54.K Kt to K 2
5.P to Q 45.P takes P
6.Kt takes P6.Castles
Even game.
FOUR KNIGHTS' GAME.

If Black moves 3. ... Kt to B 3, the previous moves being the same as in the Three Knights' Game, it is called the Four Knights' Game. It is convertible into a Ruy Lopez, if White plays 4. B to Kt 5; and into a Double Ruy Lopez, if Black replies 4. ... B to Kt 5.