The yard is the basic pre-metric unit of length in the English-speaking world, since 1959 in the United States and since 1963 in the United Kingdom exactly equal to 0.9144 meter (see international yard).
Since at least the 12th century the yard has been subdivided into 3 feet and 36 inches. It was formerly (before 15th century - ) also subdivided in a binary fashion, mainly by clothmakers, the chief divisions being 4 quarters and 16 nails (nayles).
For earlier values of the yard, see the history sections below.
In many trades, = cubic yard. Used for commodities like concrete and soil.
In his Chronicles William of Malmesbury (1095 – 1143?) tells how the “false yard” was corrected by referring it to the length of King Henry I's arm. The story is not just a legend–William's descriptions of contemporary events are reliable–but William does not say this was the origin of the yard; it existed before Henry I was born. The yard was in the keeping of the guilds that dealt in cloth.
On 20 November 1196, Richard I proclaimed an Assize of Measures, and afterwards had yard standards in the form of iron rods distributed throughout the country. The expression “by the King's iron rod,” referring to the yard, appears frequently in the records.
The yard standard of Elizabeth I, made in 1588, is still in existence and may be seen in the Science Museum in London. It consists of an iron bar with a square cross section, about ½ inch on a side. The yard is the distance between the ends of the bar. Although it was broken and repaired sometime between 1760 and 1819, it is only about 0.01 inch shorter than today's yard.
From Elizabeth I to the 18th century not much was done about the British standards of length and mass, because not much needed to be done. Then the emergence of new science and technologies (in iron and textiles, for example) began to create an awareness of the possible effect of “high-tech” measurement on national well-being.
In 1742, the Royal Society in London arranged an exchange of the standards with the Royal Academy of Sciences in Paris. To accomplish this, two identical brass bars were made and ruled lengthwise with three lines. The length of the yard, taken from a yard measure made in 1720 that was based on that of Elizabeth I, was marked off on one of the lines and labeled “E” (for English). The bars were then sent to the French, who marked off the length of half a toise on one of the other lines, labeled it “F,” kept one bar and sent the other back. The following year the Society decided to compare a number of the existing yard measures, and in the process of doing this the length of Elizabeth's yard was engraved on the third line and marked “EXCH” (for Exchequer). The E length turned out to be 0.0075 inch longer than the EXCH length. This bar is known as Royal Society bar No. 41.
About a decade later the government itself decided to look into the status of the country's weights and measures, and set up a 63-member committee under Lord Carysfort to do so. They rejected Elizabeth's standards as “very coarsely made...bent...very bad standards” but approved of the more modern design of Royal Society bar No. 41, and on the advice of experts decided to make a standard to an even more modern design, taking its length from the “E” line on the Royal Society bar. The standard was duly made in 1758 by a Mr. Bird, an instrument maker, and deposited with the Clerk of the House of Commons. None of the Carysfort Committee's recommendations were acted upon. At the request of a successor committee, Bird made a second, similar, yard standard in 1760.
There followed a succession of committees making various recommendations and suggesting various standards, culminating in the act establishing imperial measure from May 1, 1825. This act declared the yard Bird made in 1760 to be the prototype of the imperial yard. All length measures were to be based on it. Less than ten years later, in October 1834, the Houses of Parliament burned down, destroying both of Bird's yards.
The government than appointed a committee (1838) to oversee the construction of new standards of a yet more modern nature. The task of constructing the new yard was given to Francis Baily (1744–1844), who conducted extensive research on the best choice of alloy for the bar, and designed its form before dying. The task passed to the Reverend Sheepshanks.
By this time, it was possible to measure a standard's length with a precision of one part in ten million. To give some idea of the care taken in constructing the new yard standards: to account for thermal expansion of the metal, new thermometers were constructed accurate to a hundredth of a degree. To eliminate the effect of bending, the standards were measured floating in a pool of mercury.
Sheepshanks began by constructing some standards which he compared with existing yard standards that had been compared with the Bird yard. He concluded that his bar no. 2 was 36.00025 inches of the average value of those yards, and hence of the true yard, and the commission agreed. Forty new standards were then made to Baily's design and meticulously compared both with bar no. 2 and to one another, to find the one that would be ³⁶⁰⁰⁰⁰⁰⁄₃₆₀₀₀₂₅ ths of bar no. 2. This task took years.
Finally one bar was selected to be the prototype. The four next best became “Parliamentary standards,” and the remaining 35 were distributed to various cities and friendly powers. In 1855 the selected bar was made the legal standard (18 & 19 Victoria, c 72 s 2). Sheepshanks had died the day before.
For the next development, see international yard.
The yard was recognized as the basic unit of length in the United States when it was adopted for customs purposes by the Treasury Department in 1832. It was defined as the distance between the 27th and 63rd inches on a certain brass bar made by Troughton of London, who were instrument makers. This standard had been brought to the United States in 1815.
In 1856 the British Government gave the United States two of the new English standards (see history of the yard in Great Britain) of the imperial yard, one of bronze (“Bronze Yard No. 11”) and one of iron (“Low Moor Iron Yard No. 57”). Comparison of Bronze Yard No. 11 with the standard the United States had been using, the Troughton scale, showed that the latter was 0.00087 inch longer than the British imperial yard. Following its policy of basing U.S. units on British prototypes, the U.S. began to base its yard on Bronze Yard No. 11.
On April 5, 1893, T. C. Mendenhall, Superintendent of Weights and Measures, announced with the approval of the Secretary of the Treasury that U.S. standards of length would henceforth be based on the international meter.¹
In the Act of July 28, 1866 that made the use of the metric system permissible in the United States, Congress had defined the meter in terms of the inch, 1 meter = 39.37 inches. To base the yard on the meter Mendenhall turned this relation around, making the yard equal to ³⁶⁰⁰⁄₃₉₃₇ meter, or approximately 0.91440183 meter.
As a practical matter, the Bureau had already been basing its standards on the meter, especially on prototype Meter No. 27, which it had received in 1889 from the International Bureau of Weights and Measures. Meter No. 27 served as the U.S. standard of length until 1960, when the CGPM made all prototypes of the meter obsolete by redefining the meter in terms of the wavelength of light from krypton-86 (and later, in 1983, in terms of the distance light travels in ¹/₂₉₉,₇₉₂,₄₅₈ th of a second).
After Mendenhall’s 1893 order basing the yard on a prototype meter instead of a British prototype yard, the lengths of the British and U.S. yard, (and hence of the foot and inch) were different. These differences were resolved by international agreement, leading to the adoption by the United States, effective July 1, 1959, of the International Yard. This new value for the U.S. yard was about 2 parts per million smaller than the 1893-1959 U.S. yard.
1. Fundamental Standards of Length and Mass.
Bulletin 26 of the Coast and Geodetic Survey.
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Last revised: 27 July 2004.