The foot is a subsidiary unit of length in the United States customary system, defined as ⅓ yard. 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, and the foot was one third of that distance. Although in 1866 a law was passed defining the yard as 3600/3937 meter (0.9144018288 m), making the foot equal to 0.3048006096 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 foot, based on the old prototype yard, remained the standard until 1893.
1893-1959[]
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 foot based on this definition, equal to 1200/3937 meter (0.3048006096 m), was official in the United States until 1959. This foot, slightly longer than the foot which has been in use as a standard since July 1, 1959, is still in use for some mapping purposes as the survey foot.
Since 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 foot equal to exactly 0.3048 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.
Relation to other U. S. customary length units[]
The foot-pound-second system of engineering units[]
Although the foot is officially a subsidiary unit under United States law, and the yard is the primary standard unit of length, for engineering purposes it has been normal to treat the foot as the primary length standard and build systems of mechanical units on it. Two different systems have been defined for measurements based on United States customary units: the foot-pound-second absolute system, in which the pound is a unit of mass, and the foot-pound-second gravitational system, in which the pound is a unit of force (or weight).