What is Robertson screw?

A Robertson screw, also known as a square screw or Scrulox, is a type of screw with a square-shaped socket in the screw head and a corresponding square protrusion on the tool. Both the tool and socket have a slight taper. Originally designed to enable the screws to be made using cold forming of the heads, the taper has two other advantages which have helped popularize it: It makes inserting the tool easier, and helps keep the screw on the tool without the user having to hold it there. The Robertson screw is specified as ANSI Type III Square Center.