| The Allen Wrench was designed by Allen Manufacturing Co. of Hartford, Conn in 1943 (or thats when Webster lists it as entering the English language). Today, the Allen Wrench is still made by Allen Manufacturing, however many other companies have taken to making similar hex wrenches. Still, the name "Allen Wrench" refers to this style of wrench. It is often called a 'hex wrench', 'Allen key', 'hex key' or 'unbrako key' (Unbrako being the maker of Allen Keys in the UK and Australia). The 'key' usage is standard in British English and understood in American English.
The wrench itself appears to be an 'L' shaped object a few inches long with a cross section that is a hexagonal of a paticular size. There are variations of the wrench that are called 'S' wrenches and look like a straightened out 'S'. Often the 'S' wrench will have different sizes on the ends. Many times, the 'keys' can be purchased in a set that folds out of a Swiss Army Knife like handle. The 'L' shaped version has the advantage of being able be used for more torque with a longer handle or can more eaisly fit in narrow spaces with the shorter end as the handle.
Key sizes traditionally range from 1/16" to 3/8", though larger and smaller keys can be found, along with metric versions.
The advantage in manufacturing with respect to the Allen Wrench is that they are cheap to send along unassembled objects (for example with bicycles) and are easily purchased if they should get lost.
In addition to the cheapness and universal nature of the Allen Wrench, they are very easy to repair when the end becomes blunted - just sharpen it upon a grinder (taking care not to get it too hot and lose its shape).
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