King and Pawn Versus King: The Rule of the Square
The Simplest Winning Material
A king and one pawn against a lone king is the most basic winning attempt in chess. Sometimes the pawn marches in and becomes a queen; sometimes the defending king catches it and the game is a draw. Knowing which is which, before you move, is a real skill.
The Rule of the Square
The rule of the square lets you check, at a glance, whether a king can catch a passed pawn without help from its own king. Picture a square on the board: one side runs from the pawn to its queening square, and the square extends that same number of files sideways toward the defending king.
If it is the defending king’s move and that king is already inside the square (or can step into it), it will catch the pawn and draw. If the king is outside the square and cannot enter, the pawn promotes. One caution: a pawn still on its starting square can move two squares, so measure the square from the rank it can reach.
When the Kings Are Both Involved
If the attacking king escorts the pawn, the rule of the square no longer tells the whole story. Now the result usually turns on the opposition and whether the stronger side’s king can get in front of its pawn. As a guide: the defense draws if its king can reach the queening square or plant itself directly in front of the pawn; otherwise the attacker wins.
Practice the Habit
In your own games, whenever a passed pawn appears, draw the square in your mind first. It turns a tense calculation into a one-second judgment and stops you from chasing pawns you can never catch — or letting a winning pawn slip away.