Or hex key. Metric and English here. Now singly ubiquitous, it seems, in products (particularly furniture) that come “Some assembly required.”
From "Hex key" » History, at Wikipedia: “The scant documentation available indicates that the idea of a hex socket screw drive was probably conceived as early as the 1860s to the 1890s, but that such screws were probably not manufactured until around 1910. Rybczynski (2000) describes a flurry of patents for alternative drive types in the 1860s to the 1890s in the US, which are confirmed to include internal-wrenching square and triangle types (that is, square and triangular sockets) (U.S. Patent 161,390), but he explains that these were patented but not manufactured because of the difficulties and expense of doing so at the time. P. L. Robertson, of Milton, Ontario, Canada, first commercialized the square socket in 1908, having perfected and patented a suitable cold forming method, using the right material and the right die design.
“In 1909–1910, William G. Allen too patented a method of cold-forming screw heads around a hexagonal die (U.S. Patent 960,244). Published advertisements for the ‘Allen safety set screw’ by the Allen Manufacturing Company of Hartford, Connecticut, exist from 1910. Although it is unlikely that Allen was the first person to think of a hex socket drive, his patent for a manufacturing method and his realized product appear to be the first.”
Retired economics professor (“dismal scientist”). Married 40+ years to the love of my life; we have two grown daughters, both married, two granddaughters and a...