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The rule of the square

How to tell in a second whether a king will catch a passed pawn: picture a square from the pawn to its promotion square

  • Amateur
  • Terms
  • 2 min read
Contents
  1. How to build the square
  2. Example
  3. When the rule does not work

The rule of the square is a simple way to find out whether a king can catch a passed pawn when nothing is in its way. There is no need to count moves — it is enough to picture a square.

How to build the square

  1. Draw a line from the pawn to its promotion square.
  2. Build a square on this line towards the king.
  3. If the king stands inside the square or can step into it on its move, it will catch the pawn. If not, the pawn promotes.

If the pawn is on its starting square (the second rank for White), build the square as if it had already made its two-square first move — from the third rank.

Example

a8b7c6d5e4f3g2h1

1… 2. 3. 4.

The square of the b4 pawn runs from b4 to f8. The black king on f5 is inside the square and catches the pawn in time.

The square of the b4 pawn: its sides run from b4 to b8 and from b4 to f4, giving the square b4–f4–f8–b8. The black king stands on f5 — inside. With Black to move it catches the pawn.

From g5 the king would also make it: with its move it steps into the square. From h5, though, it would not: even after moving it remains outside the square, and the pawn promotes.

When the rule does not work

  • If there are pieces or pawns in the king's path.
  • If the stronger side has other pieces that help the pawn.
  • If the stronger side's move lets them “shift” the square: after a pawn move the square becomes smaller.

The rule of the square is one of the first techniques studied in pawn endgames. Together with the opposition it helps you assess simple endings quickly.