Four Half Chess
Four-player chess variant played on four half-chessboards.
Any two of the half-chessboards can be joined up to make a regular move on this full chessboard.
The game is played in two teams: Yellow and Red play against Green and Blue. Move order: Yellow, Green, Red, Blue.
The teams win and lose together. The game ends at the first checkmate, and the team whose member delivered the mate wins. The teammates cannot capture each other's pieces.
Details: Four-player chess.
The board consists of four parts, each of them is a half of a regular chessboard. Despite of the visual arrangement of them, they are semantically equivalent. In the game, the order of the parts can be arbitrarily rearranged by the buttons below the board. However, this does not affect the behaviour: the half-boards are always connected in all six possible ways, and pieces can always move from any half to any other if the move is legal on the connected regular chessboard.
These four diagrams show the same game position from different viewpoints. No matter how you view it the blue bishop can capture the yellow pawn and the red knight, but from some viewpoints it is not obvious. (Note that it cannot capture the green knight as Blue and Green are teammates.)
These three diagrams also show the same position. The yellow king cannot move into check, but from some viewpoints it is not obvious which piece gives check from where. (The red queen does not give check since Yellow and Red are teammates.)
The original, general version: Ralph Betza around 1980[ChV.com]; the rules for four-player version: Uray M. János, 2013.