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The Square Rule

The square rule tells you whether a king can catch a passed pawn. Draw a square from the pawn to the last rank and check the enemy king.

King inside the square

White pawn is on e4 and Black king is on g6. The square from e4 to the last rank covers files e to h and ranks 4 to 8, and g6 lies inside it, so the king can catch the pawn.

FEN: 8/8/6k1/8/4P3/8/8/4K3 w - - 0 1

a b c d e f g h
8 . . . . . . . . 8
7 . . . . . . . . 7
6 . . . . . . k . 6
5 . . . . . . . . 5
4 . . . . P . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . K . . . 1
  a b c d e f g h

King outside the square

White pawn is on e4 and Black king is on a8. The square from e4 covers files e to h, and a8 lies outside it, so the pawn will promote.

FEN: k7/8/8/8/4P3/8/8/4K3 w - - 0 1

a b c d e f g h
8 k . . . . . . . 8
7 . . . . . . . . 7
6 . . . . . . . . 6
5 . . . . . . . . 5
4 . . . . P . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . K . . . 1
  a b c d e f g h

Pawn moves and the square shrinks

White pawn moves from e4 to e5. The square from e5 is smaller, because the pawn is one step closer to promotion.

FEN: k7/8/8/8/4P3/8/8/4K3 w - - 0 1

a b c d e f g h
8 k . . . . . . . 8
7 . . . . . . . . 7
6 . . . . . . . . 6
5 . . . . . . . . 5
4 . . . . P . . . 4
3 . . . . . . . . 3
2 . . . . . . . . 2
1 . . . . K . . . 1
  a b c d e f g h

Key points

  • Draw a square with the pawn as one corner and the last rank as the far side. The square has sides as long as the pawn's distance to promotion.
  • If the enemy king stands inside the square, it can catch the pawn.
  • If the enemy king stands outside the square, it cannot catch the pawn, and the pawn will promote.
  • The square is drawn on the move. If it is your move, count the square after your pawn move.
  • The square rule works for a lone passed pawn with no other pieces helping either side.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Rust Playground

Exercise

White pawn is on e4 and Black king is on a8. Can the king catch the pawn?