Fog of War chess

Fog of War chess hides a different part of the board from each player. It is a game of imperfect information, meaning that neither participant knows the complete position during play. A player sees their own pieces and the squares those pieces can legally move to; everything else remains under the fog.

An opposing piece becomes visible when it occupies a square that one of the player's pieces can reach. Moving or losing a piece can change the visible area immediately. An enemy last seen on one square may therefore have moved somewhere inside the hidden region.

The king and the end of the game

Check and checkmate do not exist. Check would announce that the king is attacked, and checkmate would mean that the attack has no legal reply. Both concepts could reveal information the player is not entitled to know. A king may instead move onto an attacked square, and the game ends when one side captures the opposing king.

Most other movement rules remain familiar. The central challenge is separating knowledge from inference. A line that appears quiet may conceal an attacker, and the king can be captured after entering a square controlled by a piece the player could not see.

Jens Bæk Nielsen and Torben Osted proposed the variant in 1989 under the name Dark Chess. Online software can easily display only the information available to each player. Physical play needs an arbiter or another system that keeps the two views separate.

It differs from Kriegspiel. Fog of War gives each player a changing partial view of the board. In Kriegspiel, players normally see only their own pieces and rely on limited referee announcements about legality, captures, and checks.