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The United States still, for most purposes, uses the traditional, non-metric units. For length or distance, they were originally based on a prototype yard which was intended to be identical to the one used in the United Kingdom. Although in 1866 a law was passed defining the yard as 3600/3937 meter, since the law did not disestablish the prototype that had been in use, it is considered that the 1866 statute did not change the standard, and in fact defined a United States standard meter which was not precisely equal to the meter used elsewhere (though the difference was never measured to be enough to matter, using the instruments of the day). In 1893, an order by Thomas C. Mendenhall redesignated the yard as 3600/3937 m, but it is generally understood that, unlike the 1866 statute, the Mendenhall Order defined the yard in terms of the internationally recognized meter.

Platinum-Iridium meter bar

The U.S. Standard Meter, in use until 1960: many U.S. customary units are defined in terms of the SI meter.

This yard = 3600/3937 m = 0.9144018288 m was the standard of length in the United States until July 1, 1959, when a new definition took effect as a result of an agreement with the nations of the Commonwealth of Nations. The yard was reduced to exactly 0.9144 m, longer than the previous United Kingdom value, but equal to that which was current in Canada. The United Kingdom implemented this agreement in 1963, with a Weights and Measures Act of that year.

Since a number of surveys had been performed using the older standard units, the survey foot was retained as a specialized unit = 1200/3937 m = 0.3048006096 m, only for the purposes of mapping places that had been surveyed under the older standards.

The length units of the U. S. customary system (identical, since 1963, with the British Imperial system) are as follows:

Name of unit Value in terms of smaller units Value (post-1959) in terms of the meter
inch 0.0254
foot 12 inches 0.3048
yard 3 feet 0.9144
rod 5½ yards 5.0292
furlong 40 rods 201.168
mile 8 furlongs 1609.344
league 3 miles 4828.032

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