The head of a Robertson screw is provided with a square inner square. In Europe, this design has always been rare in Canada and the U.S., it is still frequent.
Named is the Robertson - drive according to its inventor, Canadian Peter L. Robertson, these screws and the appropriate screwdriver in 1908 invented and filed for a patent. The inner square allows a good adhesion. In contrast to the then usual screws with the screwdriver does not slip so easily out of the screw head. Thus, this system was better suited for industrial use.
This also convinced the Ford Motor Company, one of the first customers of Peter L. Robertson. The Ford Model T 700 Robertson screws were used.
The output of the screwdriver is a square head. Similar to Allen, there is the Robertson screwdriver with angled screwdriver blade.