An imaginary box from a passed pawn to its promotion square. If the defending king can step inside it, the pawn is caught.
Why it matters
The square rule tells you in one look whether a king can catch a runner. Draw an imaginary box with the pawn in one corner and the promotion square below it: if the defending king can step into that box, the pawn is caught.
It saves calculation and it is exact. In a race with seconds on the clock, this is the difference between finding the answer and guessing it.
See it on a board
The black king is inside the pawn's square, so it arrives exactly in time: the pawn queens and is taken on the same square. With only kings left the game is drawn.
Checked: this line reaches a drawn position.
How to play it
- Count the squares from the pawn to promotion. That is the side of the box.
- Draw the box towards the defending king and see whether the king is inside it, or can step inside on its move.
- Remember the pawn on its starting square moves two, so the box is measured from where it can reach.
At each level
The same idea, said the way it is taught at each age.
Pawn · ages 3–6
Draw a box from the pawn to the last row. If the king can step into the box, it catches the pawn.
Knight · ages 4–6
Watch for the two-square first move, and for your own pawns getting in the king's way. Both break the count.
What goes wrong
- Forgetting that a pawn on its starting square can move two squares, which shrinks the box.
- Counting the box while ignoring a pawn or piece that blocks the king's path.
Questions people ask
- Does the rule still work if the pawn has support?
- No. The square rule is only about a lone pawn and a lone king. Once another piece can help either side, you have to calculate.