Hex Key

A hex key, also called a hex wrench or Allen key, is a simple tool for fastening and loosening bolts or screws that have an internal hexagonal socket in the head. The tool is a single rod of hard hexagonal steel with blunt ends that fit snugly into a matching screw socket, usually bent into an L shape with two arms of different lengths, and a user normally grips the longer arm to apply torque at the shorter end, though the short arm can be turned instead when the screw is hard to reach or needs faster rotation. William G. Allen patented a cold forming method for making hexagonal socket screw heads in 1909 and 1910, and the Allen Manufacturing Company of Hartford, Connecticut was advertising its Allen safety set screw by 1910; Allen remains a registered trademark, now owned by Apex Tool Group, though the name became genericized, and other countries use their own names for the same tool, including INBUS in Germany, Unbrako in Scandinavia and brugola in Italy. Hex keys are made to tight tolerances, usually sold in graduated sets, and are inexpensive, portable and protect a bolt head from damage because the tool meets the fastener across six contact surfaces rather than two or four, making them common for assembly and repair work across many industries.