Luft is the German word for 'air': a small pawn move in front of a castled king (such as h3 or g3) that opens an escape square and guards against back-rank mate.
A castled king behind an unmoved pawn shield is safe from most attacks but vulnerable to one specific disaster — a rook or queen arriving on the back rank, where the king's own pawns leave it no flight square. That is the back-rank mate.
Making luft solves it: nudging the h- or g-pawn one square forward (h3, h6, g3 or g6) gives the king a hole to step into, so a check along the first rank is no longer mate. Spending a single quiet tempo on luft has rescued countless games.
The trade-off is that the pawn move slightly loosens the king's shelter and can create a target, so make luft when a back-rank threat is real rather than reflexively. Many players prefer the h-pawn because it weakens the shield least.
Pushing a pawn in front of your castled king (usually h3/h6 or g3/g6) one square, so the king gains an escape square and can't be mated on the back rank.
Not automatically — the pawn move slightly weakens your king's shelter. Make luft when a back-rank mate is a genuine threat, and the h-pawn is often the safest pawn to nudge.
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