An Allen wrench is a particular type of six-sided, "L" shaped "hex key" tool that is used to tighten or loosen a hexagon screw. Allen wrenches come in various sizes and have been patented all over the world under several different names.
The first recorded developments of the tool that would eventually be called an Allen wrench were performed in 1911 by the Standard Pressed Steel Company (SPS) in Philadelphia. According to Museum Stuff, SPS developed this internal-wrenching hexagon drive as a response to the traditional internal-wrenching square drive that was popular in England at the time, but was also already patented.
Origin of Name
Most people became familiar with the Allen wrench around the time of World War II, when industrialization was in full swing. The Allen Manufacturing Company in Hartford, Connecticut patented their hexagon wrench in 1943, and the product became so successful that most consumers now simply refer to it as an Allen wrench.
According to Museum Stuff, the Allen wrench has been patented all over the world under various different names. In many European countries, for example, the Allen wrench is known as an "Inbus" due to its patent by a German manufacturer of that name.