Key squares are the squares your king needs to reach to guarantee promoting a pawn — get the king onto one and the pawn queens, no matter what the opponent does.
In a king-and-pawn ending, the pawn alone can’t force its way home — the king has to escort it. The key squares are the specific squares from which the king controls enough of the board to shepherd the pawn to promotion against any defense.
For a pawn on the 2nd to 4th rank the key squares are the three squares two ranks ahead of it; for a pawn on the 5th rank or beyond, simply reaching the squares beside and in front of it does the job. Get your king to a key square with the opposition and the win is automatic.
Thinking in key squares turns vague king-and-pawn play into a concrete target: don’t just push the pawn — race the king to the square that wins.
For a pawn on its 2nd–4th rank, count two ranks forward and take that square plus the one on either side — three key squares. Reaching any of them with your king (and the opposition) wins.
Less so — a lone rook pawn (a- or h-file) can’t use the same rule, because the defending king often reaches the corner and draws regardless of key squares.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.