The rule of the square is a visual calculation technique used in endgames to determine, without counting move by move, whether a king can catch a passed pawn before it promotes to a queen.
The principle is straightforward: mentally trace a square whose side length equals the number of moves remaining before promotion (counting from the pawn's current square to the promotion square, inclusive). If the opposing king is inside that square — or can step into it on their turn — they catch the pawn. Otherwise, the pawn promotes freely. Note that the side to move matters: if the pawn moves first, trace the square from its next square rather than its current one.
To build the square, count the remaining moves to promotion. For example, a white pawn on e5 needs 3 moves to reach e8 (e6, e7, e8). The square therefore spans e5–h5–h8–e8, covering a 4×4 zone (4 files × 4 ranks). If the black king is outside this square and it is White’s turn, the king cannot stop the pawn.
In practice, this rule saves you valuable time during time pressure or tense endgames. As soon as a passed pawn appears on the board, visualise the square immediately — it gives you a reliable, instant answer about the race between the king and the pawn.