5/16 inch (8 mm) diameter truss rod wrench. Strong carbon steel shaft with magnetic screwdriver. The ergonomic easy-grip non-slip grip allows you to work comfortably. It is also equipped with a protective bumper to prevent the head from scratching when turning the truss rod. Matches guitars with 5/16" diameter truss rods such as Gibson, PRS (USA), Gretch and current GUILD USA.