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King David I of Scotland in his Assize of Weights and Measures (c. 1150) is said to have defined the Scottish inch as the width of an average man's thumb at the base of the nail, even including the requirement to calculate the average of a small, a medium, and a large man's measures.^{[13]} However, the oldest surviving manuscripts date from the early 14th century and appear to have been altered with the inclusion of newer material.^{[14]}
Charles Butler, a mathematics teacher at Penny Cyclopædia, observing that standard measures had since surpassed the barleycorn definition of the inch, and that to recover the inch measure from its original definition, in the event that the standard measure were destroyed, would involve the measurement of large numbers of barleycorns and taking their average lengths. He noted that this process would not perfectly recover the standard, since it might introduce errors of anywhere between one hundredth and one tenth of an inch in the definition of a yard.^{[17]}
The now obsolete Scottish inch (Scottish Gaelic: òirleach), 1/12 of a Scottish foot, was about 1.0016 imperial inches (about 2.5441 cm).^{[18]} It was used in the popular expression Gie 'im an inch, an he'll tak an ell, in English "Give him an inch and he'll take an ell", first published as "For when I gave you an inch, you tooke an ell" by John Heywood in 1546.^{[19]} (The ell, equal to 37 inches (about 94 cm), was in use in England until 1685.)^{[20]}
Before the adoption of the metric system, several European countries had customary units whose name translates into "inch". The French pouce measured 2.70 cm, at least when applied to describe the calibre of artillery pieces (see also: Units of measurement in France). The Amsterdam foot (voet) consisted of 11 Amsterdam inches (duim) (see Dutch units of measurement). The Amsterdam foot is about 8% shorter than an English foot.
The current internationally accepted value for the imperial and US customary inch is exactly 25.4 millimetres.^{[21]} This is based on the international yard of exactly 0.9144 metres adopted through the International yard and pound agreement in 1959.^{[22]} Before the adoption of the international inch various definitions were in use. In the United Kingdom and most countries of the British Commonwealth the inch was defined in terms of the Imperial Standard Yard. The United States adopted the conversion factor 1 metre = 39.37 inches by an act in 1866,^{[1]} and in 1893 Mendenhall ordered the physical realization of the inch be based on the international prototype metres numbers 21 and 27, which had been received from the CGPM together with the previously adopted conversion factor.^{[23]}
In 1930 the British Standards Institution adopted an inch of exactly 25.4 mm. The American Standards Association followed suit in 1933. By 1935 industry in 16 countries had adopted the "industrial inch" as it came to be known.^{[24]}^{[25]}
In 1946 the Commonwealth Science Congress recommended a yard of exactly 0.9144 metres for adoption throughout the British Commonwealth. This was adopted by Canada in 1951.^{[26]} The United States, the United Kingdom, Canada, Australia, New Zealand and South Africa signed a treaty agreeing to the same standards on 1 July 1959.^{[27]} This gives an inch of exactly 25.4 mm. However, the United States retains the 1/39.37-metre definition for survey purposes creating a slight difference between the international and US survey inches; the difference is exactly 2 millionths of the US survey inch.^{[28]} This is approximately 1/8-inch in a mile.
International System of Units, Interferometry, Imperial units, Metre, Metric system
French Revolution, International System of Units, France, Paris, Units of measurement
Din, Torque, Motive power, Power (physics), Kilowatt
Metre, Foot (unit), Inch, Santo Domingo, Duarte Province
Second, Ton, Foot (length), Minute, Water
Millimetre, Inch, Michelin, Dallara, Kilowatt
Metre, Foot (unit), Inch, Netherlands, Limburg (Netherlands)