The rule of the square

The rule of the square tells you whether a king can catch a passed pawn without calculating every move. Draw a square from the pawn to its promotion square, using the side toward the king.

1. If the pawn is on a4 and it promotes on a8, that is four steps. The square runs four files toward the enemy king, to the e-file, and four ranks up to the eighth.
2. If the enemy king is inside that square on its move, it can catch the pawn. If it is outside, the pawn promotes, as long as nothing blocks the pawn and the pawn is not blocked from moving.
3. If the pawn is still on its starting square, count the square as if it has already moved one square, because the first move can be two squares.
4. In the line below the black king starts on h8, outside the square of the pawn on a4. The pawn promotes. The white king on h1 does not take part.

On the board