Patrol chess
Encyclopedia
Patrol chess is a chess variant
Chess variant
A chess variant is a game related to, derived from or inspired by chess. The difference from chess might include one or more of the following:...

 in which captures can be made and checks given only if the capturing or checking piece is guarded (or patrolled) by a friendly unit. Non-capturing moves are played as normal.

The variant was invented by Frederik Hendrik von Meyenfeldt who published a chess problem
Chess problem
A chess problem, also called a chess composition, is a puzzle set by somebody using chess pieces on a chess board, that presents the solver with a particular task to be achieved. For instance, a position might be given with the instruction that White is to move first, and checkmate Black in two...

 using the rules in The Problemist (the magazine of the British Chess Problem Society) in 1975.
The diagram position shows some of the peculiarities of patrol chess. The white king cannot take the black knight because it is not guarded by a friendly piece. Similarly, the black rook is not giving check, and neither is the white knight on f7. If white were to play Nbd8 (see algebraic notation
Algebraic chess notation
Algebraic notation is a method for recording and describing the moves in a game of chess. It is now standard among all chess organizations and most books, magazines, and newspapers...

), however, this would be check, as the knights would patrol one another. Black could reply Ke4, with checkmate: the king patrols the rook on e3 giving check and guarding b3 and d3, the rook on a2 is patrolled by the knight on b4 and so controls the squares b2, c2 and d2, Kd4 is not possible because the black king, patrolled by the e3 rook, controls that square, and Kxb4 is not possible because the white king is not patrolled and so cannot capture.

Patrol chess has often been used as a condition in chess problem
Chess problem
A chess problem, also called a chess composition, is a puzzle set by somebody using chess pieces on a chess board, that presents the solver with a particular task to be achieved. For instance, a position might be given with the instruction that White is to move first, and checkmate Black in two...

s. It is also possible to play complete games under patrol chess rules.

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