A jack is something you fly on the back of a boat or ship (on a jackstaff). There are at least four local jacks that I can think of, eg the Army one which is r

A jack is something you fly on the back of a boat or ship (on a jackstaff). There are at least four local jacks that I can think of, eg the Army one which is red with a union flag motif in the upper left next to the staff. The RN has a white background with the union flag motif in the same place. Merk Navy - dark blue, RAF - light blue. The Union flag is quite odd and in my opinion a masterpiece. It's not a jack.