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, (and hence the foot, inch and every other linear unit) were slightly different.
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 (The international yard is about 2 parts per million smaller than the 1893-1959 U.S. yard.) The British adopted it 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, these changes 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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