Zugzwang

Chess does not allow a player to pass. Zugzwang occurs when a player would keep an acceptable position if it could remain unchanged, but every legal move makes it worse. Moving may lose a pawn, abandon a decisive square, or let the opposing king enter. No move preserves the situation.

Imagine two kings protecting blocked pawns. The player who moves first must step away, allowing the opponent to advance or capture. If the same arrangement harms either player whenever it is that player's turn, it is mutual zugzwang. Changing only the turn also changes who is at a disadvantage.

Why it often appears in endgames

Zugzwang is common in endgames, the stage with few pieces left, because there are fewer harmless waiting moves. A tempo is one move's worth of time. In these positions, making the opponent use that tempo can decide the result.

Changing who must move

• In the simplest form of opposition, the kings face each other with one square between them. The player who must move the king has to step aside.
• Triangulation is a three-move route that returns a piece to its starting square and recreates the position with the turn changed.

Opposition and triangulation can be used to hand the turn to the opponent and produce zugzwang.

What zugzwang is not

Zugzwang does not mean that only one move is available or forced. Several legal moves may exist, but every one makes the position worse. It is also different from stalemate: in stalemate, the player has no legal move and the king is not in check, so the game is drawn. A stalemate resource aims for that result.