Passed pawns and the square rule
A passed pawn has no enemy pawns ahead of it on its own file or the adjacent files. It can reach the last rank and promote to a piece. However, the opposing king may stop it. The square rule helps assess a race quickly in a simple pawn ending. After making that assessment, still check the actual moves on the board.
In this article
Draw the square to promotion
A white pawn on a4 needs four moves to reach a8: a5, a6, a7 and a8. Draw a square with corners at a4, e4, e8 and a8. If the defending king can enter that square on its next move, it can usually catch an unsupported pawn. Always account for whose turn it is: after the pawn advances, the square becomes smaller.
Draw the square to promotion
f4–e5: the black king approaches but remains outside the new square.
Calculate our pawn race
In the example, White starts with a4–a5. The black king moves from f4 to e5, but the new square is already bounded by a5, d5, d8 and a8. The king remains outside. The pawn then advances to a6 and a7 while the king approaches through d6 and c7. The move a7–a8 promotes the pawn to a queen: Black did not intercept it in time.
- a4a5
- f4e5
- a5a6
- e5d6
- a6a7
- d6c7
- a7a8
When the shortcut is not enough
The rule is useful for an unobstructed race where the pawn can advance and other pieces do not block the king. A blockade, checks, protection by the pawn's king and control of its route change the calculation. For a pawn on its starting rank, account for an available double move. Do not apply the square mechanically to a complex position: check each move for both sides.
Practise advancing the pawn
Replay all seven half-moves of the example and watch the distance to promotion shrink. Then restart and calculate the race without hints. A queen is often the best choice, but promotion does not always mean an immediate win: stalemate and other details can matter. Open the pawn-promotion exercise, promote the pawn and check that its destination square is safe before moving.
Rules and sources
Use these links to check the rules and explore the topic further.