Prior to 1893Edit
The original definition of the yard was based on a prototype yard which was intended to be identical to the one used in the United Kingdom, and the inch was one thirty-sixth of that distance. Although in 1866 a law was passed defining the yard as 3600/3937 meter (0.9144018288 m), making the inch equal to 0.0254000508 m, 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). This inch, based on the old prototype yard, remained the standard until 1893.
In 1893, an order by Thomas C. Mendenhall redesignated the yard as 3600/3937 m (0.9144018288 m, the same definition as before), but it is generally understood that, unlike the 1866 statute, the Mendenhall Order defined the yard in terms of the internationally recognized meter. Since no measurements of the previous prototype yard have been made to current standards of precision in terms of the official meter as it was defined in 1893 (based on the prototype meter in France), it cannot be determined how great a change took place in the length of the United States standard yard in 1893. The standard inch based on this definition, equal to 100/3937 meter (0.0254000508 m), was official in the United States until 1959.
On July 1, 1959, 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, making the inch equal to exactly 0.0254 m, a compromise which was longer than the previous United Kingdom value, but equal to that which was current in Canada. It remains currently the official unit of length in the United States.
For purposes of land survey, the pre-1959 value of the inch, link, foot, chain, and mile were retained, which differ by approximately two parts per million (they are larger) or a difference of roughly 1/8" per mile. (When it is necessary to refer to these pre-1959 values, the adjective"survey" is used, as in Survey foot. For other purposes, the differences are smaller than the tolerance on measuring equipment, so the international inch is used for all operations that don't require great precision (anything other than land survey).
Relation to other U. S. customary length unitsEdit
- 12 inches = 1 foot