Four-Player Chess is an extension of the classic game to four players. Its initial position is shown in the figure below. Players strictly alternate turns clockwise, starting with White (1) to move, then Red (2), then Black (3), then Blue (4). Most rules from the two-player game remain unchanged, and the main differences are:
- Pawns are allowed to bend-off into a new direction at the diagonals such that the distance to promotion is kept the same as normal. This allows promotions on all sides. Once a pawn has bent-off, its direction can no longer change.
- En-passant can sometimes be combined with a capture move resulting in two captured pieces.
- Players can be eliminated in two ways: either they are check-mated at the beginning of the turn, or their king is hanged. The king hanging occurs for a player when that player cannot respond to his king being put in check due to turn order.
- The winner is the last player standing.
- Esser, M., Gras, M., Winands, M.H.M., Schadd, M.P.D., and Lanctot, M. (2014). Improving Best-Reply Search. In Computers and Games (CG 2013). To appear in LNCS.