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Rule of the Square

Beginner Must Know

A quick visual method to determine if a king can catch a passed pawn.

The Rule​

Draw a square from the pawn to the promotion square. If the defending king can enter this square, it catches the pawn.

FEN: 8/8/8/p7/8/8/8/5K2 w - - 0 1

The "square" of the a5 pawn extends to a5-a1-e1-e5. Can White's king enter?

How to Find the Square​

  1. Draw a diagonal from the pawn to the promotion rank
  2. Complete the square toward the defending king
  3. If the king can step into the square, it catches the pawn

Examples​

King Catches the Pawn​

FEN: 8/8/8/p7/8/8/4K3/8 w - - 0 1

White's king is on e2. The square is a5-a1-e1-e5. Kd3 enters the square—king catches pawn!

King Cannot Catch​

FEN: 8/8/8/p7/8/8/8/6K1 w - - 0 1

White's king is on g1, outside the square (a5-a1-e1-e5). It cannot step into the square on its move (even Kf1 stays outside the e-file edge), so by the rule of the square the king cannot catch the pawn, and it promotes.

With the King Supporting​

When both kings are present, the rule becomes more complex. The supporting king can block.

Practice​

For any passed pawn position, ask:

  1. Where is the promotion square?
  2. What is the "square" of the pawn?
  3. Can the defending king enter?

Summary​

The rule of the square is a quick visualization tool. Use it to assess pawn races at a glance.