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The 'check-mate system' was a system of ship identification used by the Royal Navy during World War II. It was used by the Navy when on patrol looking for German Auxiliary cruiser and others ships that had been disguised by Axis forces.
Under this system, a patrolling Royal Navy vessel would individually identify a suspect ship via the Admiralty in London; this step became necessary and was introduced after October 1942 after a German-held British ship was twice passed off by the Germans as still being British when challenged. The system became fully operational from May 1943.