Rule of the square
The rule of the square is a shortcut for a lone pawn racing an enemy king: draw a square from the pawn's square up to its promotion square, and the king catches the pawn only if it stands inside or can step in.
Also called: square of the pawn
Counting a race move by move is slow and easy to get wrong, so let a square do the counting. Take the stretch of the file from the pawn to its promotion square as one side and extend it sideways toward the defending king into a full square. A pawn on c4 gives the square with corners c4, c8, g8 and g4.
Now look at the defending king. If it stands inside the square, or can step into it with the move it is about to make, it reaches the pawn in time. If it cannot, the pawn queens. Each pawn step shrinks the square by one rank and one file, so the check has to be repeated after every move, and whose turn it is matters as much as where the king stands.
One adjustment catches many players out. A pawn still on its starting square can advance two squares at once, so its square is measured as if it already stood one rank further up, as the diagram shows.
The rule covers only the race between the pawn and the defending king. When other pieces are on the board, or the pawn's own king can stand in the defender's way, count the moves directly instead.
Compare it with Outside passed pawn. An outside passed pawn is a passer on a distant wing that works as a decoy: the defending king has to go and stop it, and the moves this takes let the other king win pawns elsewhere.
It sits close to Pawn promotion race. A pawn promotion race is a position where both sides push passed pawns toward promotion, and the result depends on who queens first and what the new queen can do.
Also worth reading is Reti maneuver. The Reti maneuver is a king walk that heads for two goals at once, usually stopping a distant passed pawn while also supporting the king's own pawn, so the opponent cannot defend against both threats.
If "Rule of the square" has cost you half a point, a first lesson is a good place to find the pattern behind it: George Li goes through your games and builds a plan.
Related terms
A passed pawn has a clear road to promotion as far as enemy pawns are concerned: none stands ahead of it on its own file or the two neighboring files, so the opponent must spend a piece or the king to stop it.
A pawn promotion race is a position where both sides push passed pawns toward promotion, and the result depends on who queens first and what the new queen can do.
Key squares are the target squares for the attacking king in a pawn ending: once it reaches one while its pawn is safe, the pawn promotes by force, no matter whose move it is.
King activity describes how freely a king can take part in the endgame, by centralizing, attacking enemy pawns and supporting its own, and the more active king frequently decides otherwise equal positions.
An outside passed pawn is a passer on a distant wing that works as a decoy: the defending king has to go and stop it, and the moves this takes let the other king win pawns elsewhere.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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