Robertson screw

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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. The contemporary square-drive screw has all but replaced the Robertson screw proper and is commonly referred to as a Robertson because it has practically identical drive dimensions and the same colour identification system, but the contemporary square drive socket has parallel sides rather than tapered. The original purpose of the taper was to enable the screws to be made using cold forming of the heads,but the taper had two other advantages which have helped popularize it: It makes inserting the tool easier and it helps keep the screw on the tool without the user's having to hold it there. The Robertson screw is specified as ANSI Type III Square Center.