The British Imperial system of units was established on June 17, 1824, by the Weights and Measures Act. Strictly speaking, it is not correct to use the term British Imperial for units in use before that date, and they are better described as traditional British. The base unit of length or distance in the British Imperial system was the yard. The yard was defined as the distance between a pair of lines etched in gold plugs inserted in a bronze bar in the custody of the clerk of the House of Commons, which had been designated a standard yard in 1760, measured at 62 degrees Fahrenheit. The new yard was actually a standard that had been commissioned by the Royal Society in 1742, which in turn had been based on an earlier Elizabethan standard. In 1834, the burning of the Houses of Parliament destroyed this standard, which had served in an official capacity for only 9 years and 198 days, and new copies were prepared by reference to the best copies of the old standard that had been found. This standard became official in 1855. In 1878, an act conferming this standard was adopted, referring to one of those copies, kept in the Standards Department of the Board of Trade in London. This yard had a length of 0.914398416 m. (Cardarelli gives a value of 0.91443992 m, but this is in error.) For the pre-1963 lengths of the other units in this system, see the specific pages on those units.
In 1963, a new Weights and Measures Act defined the yard in terms of the metric units, conforming to an agreement with the United States, so that the value has since then agreed with the International yard of 0.9144 m, slightly larger than the earlier value (but smaller than the value current in the United States prior to unification).
In 1980, the British government began withdrawing authorization of many British Imperial units, though in 1985 the yard was reaffirmed as a standard by a new Weights and Measures Act of that year. In 1995, however, the metric system became compulsory for all official purposes in Britain.
The length units of the British Imperial system (identical, since 1963, with the United States customary system) are given in the following table:
|Name of unit||Value in terms of smaller units||Value (post-1963) in terms of the meter|