Kriegspiel (chess)

Kriegspiel is a chess variant characterized by imperfect information, meaning players do not have a complete view of the board state. It simulates the fog of war, where neither player knows the exact location of their opponent's pieces.

The core principle of Kriegspiel involves a neutral umpire who observes the full board and adjudicates moves. Players communicate their intended moves to the umpire, who then provides limited feedback based on the legality and effect of the move.

Typically, players are informed by the umpire about the following:

• Legality of the move: Whether the proposed move is legal for the piece in question, assuming it exists on a square where the move would normally be allowed.
• Checks: If the move puts the opponent's King in check. If the King is in check, the umpire will announce "Check." The player in check must declare which piece is delivering the check (e.g., "Check from pawn," "Check from knight," etc.). If the declaration is incorrect, the player loses their next turn.
• Captures: When a move results in a capture, the umpire will announce that a capture has occurred, but not necessarily which piece was captured or where it was captured. Certain conventions, however, allow limited questions.
• Illegal moves: If a move is illegal (e.g., moving a piece off the board, moving through another piece when not allowed), the umpire will announce "Illegal." The player loses their turn.
• "Nothing": If the proposed move is legal but does not result in a check or capture, the umpire may say "Nothing" (or remain silent, depending on the specific ruleset).

Players are not directly told the location of their opponent's pieces. They must deduce this information based on the umpire's announcements, the legality of their own moves, and their opponent's responses to checks.

Variations in Kriegspiel rules exist regarding additional information revealed by the umpire, such as the type of piece delivering a check or allowing questions about possible captures. The most detailed versions reduce the "fog" somewhat, but the fundamental principle of incomplete information remains.

Kriegspiel provides a significant departure from standard chess strategy, emphasizing deduction, probabilistic reasoning, and psychological play, as players try to anticipate their opponent's moves based on limited information and possible bluffs. It is generally regarded as a challenging and intellectually stimulating variant, highlighting different aspects of chess skill than standard play.