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The rule of the square

The rule of the square is a simple technique for judging instantly whether the king can hold up a passed pawn. Picture a «square» stretching from the pawn to its queening square: if the enemy king is inside it (or can step into it on the move), the pawn is caught; if the king is outside, the pawn goes through.

Here White has a pawn on a2 and the king on h1; Black has a lone king on g8. White to move, and the position is won (per the tablebases): after the dash a2-a4 the black king is outside the pawn's square and cannot catch it - it queens.

The rule of the square saves time and calculation: in a pawn race one glance settles the outcome. The subtlety to remember is that from its starting square a pawn can step two squares at once.

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Drag a piece onto the board, or click a piece and then click the squares. The cross beside the pieces sweeps every piece of that colour off the board, and a single piece can be dragged off the edge.

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How to count the square

  1. 1Picture the square in your head: its side is the distance from the pawn to the queening square.
  2. 2If the weaker side's king can step into that square on the move, the pawn is caught with no need to calculate lines.
  3. 3A pawn on the second rank can move two squares, so the square is drawn from the third rank rather than from the starting one.
  4. 4Check that your own king or a piece is not in the way of entering the square: the rule only works if the path is clear.

Common mistakes

  • Counting the square from the pawn's starting square, forgetting it may move two.
  • Relying on the rule when the king is obstructed by its own pawn or is in check - those cases need concrete calculation.
  • Forgetting whose move it is: the square is checked for the side that is to move now.

Common questions

What is the rule of the square in chess?

It is a way of telling at a glance whether the king catches a passed pawn. Draw a square whose side runs from the pawn to the queening square: if the king can step into it on the move, the pawn is held up.

Does the rule of the square work for a pawn on the second rank?

Yes, but the square is drawn from the third rank: from its starting square the pawn may move two, which shortens its journey by a tempo.

When does the rule of the square not work?

When the king is obstructed by its own pieces, when it is tied to defending another pawn, or when it runs into a check on the way. Those positions need exact calculation.

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