<P> The international standard symbol for a foot is "ft" (see ISO 31 - 1, Annex A). In some cases, the foot is denoted by a prime, which is often marked by an apostrophe, and the inch by a double prime; for example, 2 feet 4 inches is sometimes denoted as 2 ′ − 4", 2 ′ 4" or 2 ′ 4". (See' minute' for another case where prime and double prime symbols are used to denote first and second cuts in refining measurement .) </P> <P> In the United States, the foot was defined as 12 inches, with the inch being defined by the Mendenhall Order of 1893 by 39.37 inches = 1 m . In Imperial units, the foot was defined as ⁄ yard, with the yard being realized as a physical standard (separate from the standard meter). </P> <P> The yard standards of the different Commonwealth countries were periodically compared with one another . The value of the United Kingdom primary standard of the yard was determined in terms of the meter by the National Physical Laboratory in 1964 as 6999914396900000000 ♠ 0.914 3969 m, implying a pre-1959 foot in the UK of approximately 6999304798966667000 ♠ 0.304 798 966 667 m . </P> <P> When the international foot was defined in 1959, a great deal of survey data was already available based on the former definitions, especially in the United States and in India . The small difference between the survey and the international foot would not be detectable on a survey of a small parcel, but becomes significant for mapping, or when the state plane coordinate system (SPCS) is used in the US, because the origin of the system may be hundreds of thousands of feet (hundreds of miles) from the point of interest . Hence the previous definitions continued to be used for surveying in the United States and India for many years, and are denoted survey feet to distinguish them from the international foot . The United Kingdom was unaffected by this problem, as the retriangulation of Great Britain (1936--62) had been done in meters . </P>

Which is an expression of distance in si units