The rule of the square: know at a glance if a pawn can be caught
A simple visual technique for knowing, without calculating a single move, whether the enemy king can stop your pawn from promoting.
Key takeaway
Mentally draw a square from the pawn to its promotion square: if the enemy king is already inside it, it can catch the pawn.
Calculating move by move whether a king can catch an enemy pawn takes time, and a miscalculation is costly. The rule of the square settles it with a single glance, without counting a single move.
How to draw the square
From the pawn's square, count how many ranks separate it from its promotion square. Then draw a square of that size on that side of the board (as many files as ranks), with the pawn on one corner and its promotion square on the opposite corner, on the same side of the board.
The rule
If the enemy king is already inside that square, or can step into it on its move, it catches the pawn in time. If it's outside the square and can't enter it immediately, the pawn promotes freely, no matter which path the king takes toward it.
A concrete example
In the diagram above, White's pawn on e5 needs three ranks to reach e8: the square therefore stretches from e5 to h5, then up to h8 and e8. Black's king, on a8, is far outside this square and has no chance of catching the pawn, whichever route it takes.
One detail that changes everything: whose move it is
Never forget to check whose move it is before applying the rule. A king sitting right at the edge of the square can be saved or doomed depending on whether it's their move right now or they have to wait one more turn.
Key takeaways
- ✓Draw the square from the pawn's square to its promotion square, on the same side
- ✓If the enemy king is inside the square (or can step in on its move), it catches the pawn
- ✓If the king is outside and can't enter immediately, the pawn promotes with no trouble
- ✓Always check whose move it is: it can be enough to flip the outcome