Square of the pawn

The square of the pawn is an imaginary area used to compare a pawn's race toward promotion with the opposing king's route. Promotion happens when the pawn reaches the last rank and becomes a queen, rook, bishop, or knight.

To picture it, use the distance between the pawn and its promotion square as one side of the square, then extend the same distance toward the side where the opposing king stands. If the pawn still has a legal initial two-square move, imagine it one square farther forward before drawing the shape, because those two squares cost only one move. The boundary counts as part of the square.

The square is the geometric area. The rule of the square is the test that uses it to decide whether the king arrives in time. The idea is cleanest when the pawn is running unaided and no other piece blocks, checks, or changes the move count.