Late in the 19ᵗʰ century the United States stopped basing its yard on British prototypes, and defined it instead as a fraction of the meter. (See history of the yard in the United States.) Thereafter the lengths of the British and U.S. yard were slightly different, and hence so were the foot, inch and every other linear unit.
At first this difference was too small to be significant, but as machine work became more precise the difference between the British and U.S. inch became a problem. During the Second World War, for example, aircraft mechanics discovered that some parts of aircraft engines built to identical blueprints in the United States and England differed enough in size to be noninterchangeable.
To eliminate such problems, Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States agreed that effective July 1, 1959, 1 international yard = 0.9144 meter exactly.
The United States adopted this value immediately for all purposes.¹ In fact, machinists and others had been agitating for precisely this change for years, because 91.44 centimeters ÷ 36 = 2.54, making the inch exactly 2.54 centimeters. That conversion factor made working in an environment with mixed metric and inch measurement much easier, and machinists used it even before the international yard was adopted.
(The international yard is about 2 parts per million smaller than the 1893-1959 U.S. yard.)
The U.S. made an exception to the treaty for geodetic purposes; see U.S. Survey Foot. While the survey foot was in use the Geodetic Survey used the abbreviation “ift” for the international foot, to distinguish it from the survey foot (“sft”). The survey foot is to be abolished in 2022; at that time the qualifier “international” will also be dropped; all feet in the U.S. will just be feet.
The British adopted the international yard through the Weights and Measures Act of 1963, which stated that 1 yard = 0.9144 meter exactly.² The change increased the length of the yard in Britain by about one ten-thousandth of an inch.
Obviously, this change to linear measures had no effect on ordinary commerce.
Federal Register, July 1, 1959.
“Refinement of Values for the Yard and the Pound.”
Elizabeth II c 31 1963.
Public General Acts and Measures, 1963.
London: HMSO, 1963.
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Last revised: 27 May 2020.