An Allen wrench, AKA an Allen key or hex key, is a small hand tool used to fasten screws or bolts with a hexagonal socket. Either end usually works, although some sets only have one end.

Have a look: The Allen fits snugly into the recess of the screw or bolt being used, which is also six-sided. The tool is, therefore, commonly known as a hex wrench or hex key. So how did Allen ...