A hex key (also, hex wrench, Allen key and Allen wrench) is a simple driver for bolts or screws that have heads with internal hexagonal recesses (sockets).
Hex keys are formed from a single piece of hard hexagonal steel rod, having blunt ends that fit snugly into similarly-shaped screw sockets. The rods are bent to 90º, 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.
Hex keys are designated with a socket size and are manufactured with tight tolerances. As such, they are commonly sold in kits that include a variety of sizes. Key length typically increases with size, but not necessarily proportionally so