<P> The design is often criticized for its tendency to cam out at lower torque levels than other "cross head" designs . There has long been a popular belief that this was actually a deliberate feature of the design . Evidence is lacking for this specific narrative and the feature is not mentioned in the original patents . However, a subsequent refinement to the original design described in US Patent #2,474,994 describes this feature . </P> <P> Robertson, also known as a square, or Scrulox screw drive has a square - shaped socket in the screw head and a square protrusion on the tool . Both the tool and the socket have a taper, which makes inserting the tool easier, and also tends to help keep the screw on the tool tip without the user needing to hold it there . (The taper's earliest reason for being was to make the manufacture of the screws practical using cold forming of the heads, but its other advantages helped popularize the drive .) Robertson screws are commonplace in Canada, though they have been used elsewhere and have become much more common in other countries in recent decades . Robertson screwdrivers are easy to use one - handed, because the tapered socket tends to retain the screw, even if it is shaken . They also allow for the use of angled screw drivers and trim head screws . The socket - headed Robertson screws are self - centering, reduce cam out, stop a power tool when set, and can be removed if painted - over or old and rusty . In industry, they speed up production and reduce product damage . One of their first major industrial uses was the Ford Motor Company's Model A & Model T production . Henry Ford found them highly reliable and saved considerable production time, but when he couldn't secure licensing for them in the United States, limited their production use to his Canadian division . Robertson - head screwdrivers are available in a standard range of tip - sizes, from 1.77 mm to 4.85 mm . </P> <P> Reed and Prince, also called Frearson, is another historic cross-head screw configuration . The cross in the screw head is sharper and less rounded than a Phillips, and the bit has 45 ° flukes and a sharper, pointed end . Also, the Phillips screw slot is not as deep as the Reed and Prince slot . In theory, different size R&P screws fit any R&P bit size . </P> <P> Pozidriv and the related Supadriv are widely used in Europe and most of the Far East . While Pozidriv screws have cross heads like Phillips and are sometimes thought effectively the same, the Pozidriv design allows higher torque application than Phillips . It is often claimed that they can apply more torque than any of the other commonly used cross-head screwdriver systems, due to a complex fluting (mating) configuration . </P>

The flukes of a reed and prince screwdriver are at what angle