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 ...

They're so named because you need an Allen wrench (aka an Allen key or hex key) with a hexagonal socket to tighten them. The hexagonal shape provides six points to apply pressure to the fasteners ...