The rule of the square
Step 1 of 2
8
7
6
5
4
3
2
a1
b
c
d
e
f
g
h
The black passed pawn wants to run. Imagine a square from the pawn to its promotion square, as wide as it is long: here from a5 to a1 to e1 to e5. If your king is inside the square, or can step in now, it will always catch the pawn.