A hex key is a tool used to tighten and loosen a screw or bolt, that has a hexagonal depression in its head. The tool may also be called a hex wrench, Allen wrench, hex-head key, zeta wrench, Alum wrench, or Inbus key. These are used in both consumer and industrial assembly, but are often associated with self-assembly kits sold to consumers, particularly furniture kits.
The first hex key was developed in 1911. The tools came into common use during the manufacturing boom of World War II. The Allen Manufacturing Company obtained the first patent in 1943, and its model became so popular that the brand name Allen wrench is frequently used to refer to any such tool, even those manufactured by other companies.
The term 'hex' comes from hexa, the Greek word for 'six.' The tool is made of a six-sided strip of metal that is bent at a 90° angle about one-third of the way from one end. Usually, the short side is inserted into the fastener, while the long side provides a hand-hold for the user, but most hex wrenches are reversible. The user rotates the key to turn the screw or bolt. The amount of torque, or force, that the tool can apply depends on the length and thickness of the hex key.
Fasteners with hex heads are often easier to turn than those with Phillips heads or standard screwdriver slots. The six points of contact allow the hex key to fit more snugly into the fastener head than is possible with a screwdriver. This helps prevent the tool from slipping out of the fastener head. In addition, the design of hex-head fasteners makes them less likely to strip out, or distort the head, than standard or Phillips head fasteners.
Hex wrenches must be used manually, as no drill attachments are available. Further, they can be difficult to maneuver in tight places because of the space needed to turn the handle of the key. Despite these drawbacks, the tool remains a light, simple, inexpensive solution for many consumer and industrial situations.
Hex keys are often purchased in sets of varying sizes. They are packaged in self-assembly furniture kits because they are small, inexpensive to manufacture, and easy to use. Inclusion in such kits prevents the user from having to find a screwdriver or drill in order to complete the project, and is thought to increase customer satisfaction with the assembly process.