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Critical squares are the neighbouring square on the [[Pawn Spans|luff-side]] (i.e. for black pawn on c2, d2),
and the diagonal neighbouring square on the lee-side one rank back (i.e. for black pawn on c2, b3).
Only if the attacking king can reach these squares in one move, it can assist in a [[Checkmate|checkmate|Checkmate]] with the queen - otherwise it is a draw
- except the rare cases the attacking king may reduce the [[Distance|Chebyshev distance]] accordantly, giving [[Discovered Check|discovered check]] <ref>Lee-case implemented as [[Interior Node Recognizer]] in [[IsiChess]]</ref>.
==Luff case==
When the defending king supports its pawn from the "wrong" [[Pawn Spans|luff-side]], including occupying its [[Promotion Square|promotion square]], the <span style="background-color: #c0ffc0;">winning area </span> for the attacking king is enlarged by at least one move, in luff-direction even two moves.
The critical luff-side square is now one more farther (i.e. for black pawn on c2, e2), and both squares need to be reached in two moves
to either transpose to a winning [[#Lee|lee-side case]], or to assist in a checkmate on the luff-side.
