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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.
Standard square drive sizes
{| class="wikitable"; style="text-align:right;font-size:92%"
|+ Square recess dimensions
|- style="text-align:left"
! rowspan=2 colspan=2| Color
! rowspan=2 | No
! rowspan=2 | Screw sizes
! rowspan=2 | Fraction
! colspan=2 | Range
|- style="text-align:left"
! in !! mm
|-
| style="width: 5px; background-color: orange;" | || Orange
| #00 || #1, #2 || ||
|-
| style="width: 5px; background-color: yellow;" | || Yellow
| #0 || #3, #4 || ||
|-
| style="width: 5px; background-color: green;" | || Green
| #1 || #5, #6, #7 || ||
|-
| style="width: 5px; background-color: red;" | || Red
| #2 || # 8, #9, #10 || ||
|-
| style="width: 5px; background-color: black;" | || Black
| #3 || #12, 1/4 || ||
|-
| style="width: 5px; background-color: brown;" | || Brown
| #4 || 5/16, 3/8 || ||
|}
Combination screws for electrical applications
Many screws used in electrical applications (for example, a typical NEMA 5-15R, breaker screws, and conduit screws) use a combination of a slotted/Phillips/Robertson screw head. A few tool manufacturers make bits to engage this screw head better than the traditional Phillips allowing for more torque before camout, for example, the C1 and C2 from Klein and the ECX #1 & #2 from Milwaukee.