Zugzwang

Zugzwang (German for "compulsion to move", pronounced [ˈtsuːktsvaŋ]) is a situation found in chess and other games wherein one player is put at a disadvantage because they must make a move when they would prefer to pass and not move. The fact that the player is compelled to move means that their position will become significantly weaker. A player is said to be "in zugzwang" when any possible move will worsen their position.

The term is also used in combinatorial game theory, where it means that it directly changes the outcome of the game from a win to a loss, but the term is used less precisely in games such as chess. Putting the opponent in zugzwang is a common way to help the superior side win a game, and in some cases, it is necessary in order to make the win possible.

The term zugzwang was used in German chess literature in 1858 or earlier, and the first known use of the term in English was by World Champion Emanuel Lasker in 1905. The concept of zugzwang was known to players many centuries before the term was coined, appearing in an endgame study published in 1604 by Alessandro Salvio, one of the first writers on the game, and in shatranj studies dating back to the early 9th century, over 1000 years before the first known use of the term.

Positions with zugzwang occur fairly often in chess endgames. According to John Nunn, positions of reciprocal zugzwang are surprisingly important in the analysis of endgames.

The word comes from German Zug 'move' + Zwang 'compulsion', so that Zugzwang means 'being forced to make a move'. Originally the term was used interchangeably with the term zugpflicht 'obligation to make a move' as a general game rule. Games like chess and checkers have "zugzwang" (or "zugpflicht"): a player MUST always make a move on his turn even if this is to his disadvantage. Tabletop war games or role playing games have not: on his turn a player can simply decide to "wait" or "do nothing". Over time however the term became especially associated with chess.

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