The question often arises whether a King can catch a Pawn which is racing to promotion. There is a simple trick, known as the 'The Rule of the Square', to answer this question without calculating any variations.
The pawn's square is visualized by first drawing a diagonal line from the Pawn to the Pawn's promotion rank. The diagonal creates an imaginary square, half of which is shown in the diagram. If the opposing King is in this square, the King can catch the Pawn. If the King is not in the square, the chase is hopeless.
Can the Black King catch the White Pawn in the diagram? It depends on who is to move. If Black is to move, 1...Kg4 puts the King into the square and Black will catch the Pawn. The King's other legal moves would be a serious mistake. If White is to move, 1.c5 creates a new diagonal, where the corner of the square is on f5. Since the Black King cannot enter the square, the Pawn will Queen and the game will become an elementary endgame of King and Queen vs. King.
