Mutual zugzwang

Zugzwang is a position where the obligation to move causes harm: passing would be preferable if it were allowed. In mutual zugzwang, the same arrangement is unfavorable for whichever side has the move. Changing only the turn can change a win into a draw or a draw into a loss.

Only one player actually moves in the real position, so only that player suffers the immediate obligation. The word mutual describes the position's property: the other side would also have to concede something if the turn were reversed. A single spare move can therefore decide the ending.

Where it appears

Mutual zugzwang is common in endings with few pawns and active kings. The kings may contest entry squares while every pawn push consumes a reserve move. A player tries to preserve a harmless reserve move or follow a route that restores a similar arrangement with the turn reversed.

The trébuchet is an extreme pawn-ending form. The kings and pawns are placed so that the player who moves first must abandon a pawn and let the opponent win. Other mutual zugzwangs are less absolute and may change the result only between a win and a draw.