The yard is the official standard unit of length in the United States customary system. It should be noted that although the yard is officially the base unit, the foot is often taken as the base unit for comprehensive scientific and engineering systems of units. Over the course of history, it has been defined in three different ways:
Prior to 1893
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. Although in 1866 a law was passed defining the yard as 3600/3937 meter (0.9144018288 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 yard based on the old prototype 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 yard based on this definition 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, a compromise which was longer than the previous United Kingdom value, but equal to that which was current in Canada. Because this value was establishedby an international agreement, this new yard is termed the international yard. It remains currently the official unit of length in the United States.