Inch

 ( Customary Units Of Measurement In The United States ) 

The inch (symbol: in or ) is a unit of length in the imperial units and the United States customary systems of measurement. It is equal to yard or of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, anthropic units from the width of the human thumb.

Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s the inch has been based on the metric system and defined as exactly 25.4Millimetre.

"Inch" is cognate with "ounce" (), whose separate pronunciation and spelling reflect its reborrowing in Middle English from Anglo-Norman unce and ounce..

In many other European languages, the word for "inch" is the same as or derived from the word for "thumb", as a man's thumb is about an inch wide (and this was even sometimes used to define the inch). In the Dutch language a term for inch is engelse duim (english thumb). Examples include ("inch") and polze ("thumb"); ("thumb"); Danish and ("inch") tommel ("thumb"); (whence and ); ; დუიმი, ; ; ("inch") and polegar ("thumb"); ("duim"); ("thumb"); ("inch") and pulgar ("thumb"); and ("inch") and tumme ("thumb").

international inch is equal to:
     

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. However, the oldest surviving manuscripts date from the early 14th century and appear to have been altered with the inclusion of newer material.

In 1814, Charles Butler, a mathematics teacher at Cheam School, recorded the old legal definition of the inch to be "three grains of sound ripe barley being taken out the middle of the ear, well dried, and laid end to end in a row", and placed the barleycorn, not the inch, as the base unit of the English Long Measure system, from which all other units were derived. John Bouvier similarly recorded in his 1843 law dictionary that the barleycorn was the fundamental measure. Butler observed, however, that "as the length of the barley-corn cannot be fixed, so the inch according to this method will be uncertain", noting that a standard inch measure was now i.e. kept in the Exchequer chamber, Guildhall, and that was the legal definition of the inch.

This was a point also made by George Long in his 1842 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 case 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.

Before the adoption of the international yard and pound, 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. In 1893, Mendenhall Order the physical realization of the inch to 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.

As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977 mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the de facto standard for manufacturers internationally, with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his.

In 1930, the BSI Group 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,