Patrol chess is a chess variant in which captures can be made and checks given only if the capturing or checking piece is guarded (or patrolled) by a friendly unit.
The variant was invented by Frederik Hendrik von Meyenfeldt who published a chess problem 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.