Fool's mate

Fool's mate occurs when White moves the f-pawn and g-pawn very early. Those pawns begin in front of and to the right of the white king. Those moves open the diagonal leading to the king and leave the nearby squares almost undefended.

Black exploits the opening by moving the queen to h4, where it checks along the diagonal toward the king on e1. In algebraic notation, files are named a through h and ranks are numbered. The queen cannot be captured, the check cannot be blocked, and the king has no legal square, so the position is checkmate.

The diagonal appears in four plies

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

Startg4e5f3Qh4#

f2 closes the diagonal to e1, and the g2-pawn can still interpose on g3.

The route is still closed

This is only one of the possible shortest routes. In the initial position, both sides still retain the blockers that prevent the finish.

1. f2 closes the diagonal to e1, and the g2-pawn can still interpose on g3.
2. The pawn on e7 blocks the black queen's route from d8 to h4.

It is the fastest mate allowed from the initial position: Black can deliver it on the second move. It is not a forced strategy, but a punishment for two severe mistakes. Its practical lesson is that moving several pawns in front of the king can open dangerous lines.