Endgames
Rule of the square
The rule of the square estimates whether a king can catch an unsupported passed pawn without help from other pieces. The square extends from the pawn to its promotion square and the same distance sideways. If the defending king enters in time, it can stop the pawn in the simple case.
A practical example
A white pawn is on a4 and promotes on a8. Its square reaches e4 and e8. A black king on e8 is inside; with Black to move it can catch the pawn via d7 and c6.
A common mistake
Ignoring the side to move, an initial two-square pawn move or support from the enemy king. Other pieces, blocked paths and checks require concrete calculation.
Try this in your next review
Visualize the square, then calculate the race move by move. Repeat with the other side to move and with the king shifted one square.
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