The Allen Head socket drive fastener was originally developed by the SPS company and was later trademarked by the Allen Manufacturing Company. The head of a Hex Socket fastener has an inset six-sided depression that accepts a matching six-sided drive tool commonly referred to as an Allen Wrench. The Allen Wrench head nowadays comes in many configurations ranging from a simple six-sided stock of hardened steel with a 90 degree bend to a short six-sided piece of steel that is pressed into a socket drive that can be used by multiple sizes of ratchet wrenches.
Often a collection of various sizes of Allen wrench segments are mounted in a single handled tool similar to a "swiss army knife". In this flip-out configuration a specific size of Allen Wrench can be pulled out of a stored position in the tool and the handle and collection of the remaining wrenches will act as a tool handle and grip-fulcrum for ease of use during turning of Allen Hex Head fasteners. These Allen Wrench key sets are normally made up of a complete set of one specification of tool dimensions being either Standard SAE increments of English Inch measurement fractions or Metric diameter sizes. These variations are commonly referred to as 'Standard' or 'Metric' sized wrenches. One common mis-use of Allen "keys" is to fit a Metric sized wrench end into a slightly mis-matching size of a Standard fastener head (or vise versa). Using the incorrect sizing standard of wrench with a mis-matched fastener will often cause the fastener or possibly the wrench head and fastener to be stripped. Other times, a slight amount of corrosion inside the fastener head or on the wrench head will require some pressure or tapping of the tool to engage the wrench into the fastener head prior to use.
Other designs of the Allen head have innovated on the requirement for the wrench to be inserted at a perfect perpendicular angle of the wrench to the fastener head. These modified Allen Wrenches are referred to as Ball-End Allen wrenches and will allow the user to insert and turn the wrench in a fastener at a slight angle. This also allows the wrench to wobble slightly side to side as the wrench is turned. The downside of a ball socket Allen Wrench is it's strength limit and tolerance to stripping out the fastener. If the bolt is tightly torqued in place, a ball-end Allen will often strip both the wrench head and fastener interior.