A hex key is a simple driver for bolts or screws that have heads with internal hexagonal recesses (sockets).

Hex keys of various sizes

Hex keys are formed from a single rod of hard hexagonal steel, having blunt ends that fit snugly into similarly shaped screw sockets. The rod is bent to 90°, forming two arms of unequal length resembling an "L". The tool is usually held and twisted by its long arm, creating a relatively large torque at the tip of the short arm; it can also be held by its short arm to access screws in difficult-to-reach locations and to turn screws faster at the expense of torque.