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Pawn endgame

The Rule of the Square

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The rule of the square replaces a simple pawn race with geometry. Count how many moves the unsupported pawn needs to promote and construct a square of that size toward the defending king. A king on or inside the boundary can catch the pawn; a king outside cannot, unless it moves first and can enter. The shortcut assumes no other piece changes either route.

On this page
  1. How to draw the square
  2. The pawn’s first move counts
  3. When it applies
  4. Try it
  5. Keep learning

How to draw the square

The white pawn on c5 needs three moves: c6, c7 and c8. The square therefore has corners c5, c8, f8 and f5. Black’s king on f5 stands on the boundary and catches the pawn even though White moves first. A king on f4 would be outside: with White to move the pawn wins, but with Black to move 1...Kf5 enters the square and draws.

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White to move, but Black’s king on f5 is on the square’s boundary. Syzygy confirms the draw.

The pawn’s first move counts

A pawn on its starting square may advance two squares on its first move, but that still consumes one move. To construct the square, imagine the pawn one square ahead, not on the square where a double move would land. For a white pawn on c2, use c3 as the starting corner: it needs five moves to promote (c2–c4, then c5, c6, c7 and c8). Also check that the double move is legal and unobstructed.

When it applies

The rule is exact for the race between an unsupported pawn and a lone pursuing king when both routes are clear. An attacking king can shoulder the defender away; another pawn can block a diagonal; a check on promotion can alter the count. When those features appear, use key squares, opposition or direct calculation instead.

Try it

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Black to move. The king is just outside the c5–c8–f8–f5 square; enter it with 1...Kf5 and catch the pawn.
What is the rule of the square in chess?Count the pawn moves to promotion and construct a square of the same side length toward the defending king. A lone king on or inside the boundary catches the unsupported pawn; a king outside may still catch it if it moves first and can enter.
Does the side to move matter?Yes, at the boundary. With a white pawn on c5 and Black’s king on f4, White to move wins the race; Black to move draws by entering with ...Kf5. Both results were checked against Syzygy.
When does the rule of the square not work?When the attacking king can shoulder the defender away, when there is a second pawn, or when the defender’s path is blocked. Then it becomes a key-squares-and-opposition problem instead.

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