Mutual zugzwang
A mutual zugzwang is a position in which whichever player has to move is worse off, so each side would gladly pass, and the result depends entirely on whose turn it is.
Here both sides are in the same predicament. If White is to move, White loses ground, and if Black is to move, Black does. The same position is therefore good for one side or the other purely by the turn to move. This is why endgame players value tempo moves. A player who can lose a move, by triangulating with the king for example, can turn a position with the wrong side to move into one where the opponent must break the balance. Mutual zugzwangs appear most often in king and pawn endings with locked pawns, where kings have few safe squares. To find one, first list every legal move for each side in the position and check that all of them are bad. Then look for a way to reach it with the opponent to move, using spare pawn moves, a free king move or a triangulation. A classic error is to walk into the position with your own move, so count the parity of the available tempo moves before committing. Learning a handful of standard mutual zugzwangs gives you reference points for many more complex endings.
Also worth reading is Trebuchet. The trebuchet is a mutual zugzwang in which two kings stand on either side of a pair of locked pawns, each beside the enemy pawn, so whoever must move loses a pawn and the game.
Also worth reading is Corresponding squares. Corresponding squares form a map for blocked pawn endings: each square the attacking king might use is paired with the square the defending king must occupy in reply, and missing a pair lets the attacker break through.
A neighboring idea is Waiting move. A waiting move keeps your position exactly as strong as before and simply hands the opponent the obligation to move, which helps whenever every reply they have makes their position worse.
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Related terms
Zugzwang is the bind of having to move when standing still would be best: the player on move has only moves that weaken the position, and since passing is not allowed, the duty to move itself costs something.
Opposition is a standoff between the two kings on one file or rank with a single empty square between them; whoever does not have to move holds it, because the other king must give up ground.
Triangulation is a way for a king to hand the move to the opponent: it makes a three-move loop over neighboring squares and returns to its starting square, while the enemy king, with less room, cannot copy the loop.
Corresponding squares form a map for blocked pawn endings: each square the attacking king might use is paired with the square the defending king must occupy in reply, and missing a pair lets the attacker break through.
A reserve tempo is a spare, harmless move, usually a pawn move, that one side can make without damaging its position, passing the obligation to move across to the opponent.
A waiting move keeps your position exactly as strong as before and simply hands the opponent the obligation to move, which helps whenever every reply they have makes their position worse.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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