Before the mid-twentieth century, an engineer measuring a precision machine part in Manchester and an engineer measuring the same part in Detroit were using different feet. The difference was imperceptible on a construction site. On a factory line churning out interchangeable aircraft parts during the early stages of the Cold War, it was unacceptable.
Under the Mendenhall Order of 1893, the United States defined the yard as 3600/3937 meters. That made one foot equal to approximately 0.30480061 meters. Great Britain, meanwhile, tied its yard to an actual physical object: the Imperial Standard Yard, a bronze bar cast in 1844 and stored in London. Because the metal alloy in that bar was slowly losing mass and shrinking over time, the British yard kept shortening, drifting toward 0.91439841 meters. Canada had already adopted its own compromise value of 0.9144 meters in 1951.
The gap measured barely a few parts per million. Yet, as aerospace tolerances tightened to micro-inches, precision tools manufactured in the United Kingdom would bind, jam, or fail when shipped to American defense contractors. Metrologists realized that physical metal bars subjected to atmospheric changes could no longer serve as national standards. The English-speaking industrial world needed a single, shared number.
