Critical square

A critical square is a square that decisively changes the outcome of a position. If a piece, usually the king in an endgame, reaches it, that side can force a specific goal against best defense. The goal may be to promote a pawn, meaning to reach the last rank and replace it with another piece, win an enemy pawn, or enter an area the opposing king can no longer protect.

A square is not critical in isolation. Its status depends on the exact position, whose turn it is, and the goal being pursued. After a pawn move or an exchange, the important squares may change. A useful test is therefore concrete: what becomes forced after the square is occupied, and why can the opponent no longer prevent it?

In elementary king-and-pawn endings, many books use critical square and key square as synonyms. This glossary uses key square for the standard sets that secure promotion in those endings, while critical square is the broader idea. That distinction is instructional rather than universal.