The earliest known reference to the inch in England is from the
Laws of Æthelberht dating to the early 7th century, surviving in a single manuscript, the
Textus Roffensis from 1120. Paragraph LXVII sets out the fine for wounds of various depths: one inch, one shilling; two inches, two shillings, etc. An Anglo-Saxon unit of length was the
barleycorn. After 1066, 1 inch was equal to 3 barleycorns, which continued to be its legal definition for several centuries, with the barleycorn being the base unit. One of the earliest such definitions is that of 1324, where the legal definition of the inch was set out in a statute of
Edward II of England, defining it as "three grains of
barley, dry and round, placed end to end, lengthwise". One, dating from the first half of the 10th century, is contained in the Laws of
Hywel Dda which superseded those of
Dyfnwal, an even earlier definition of the inch in Wales. Both definitions, as recorded in
Ancient Laws and Institutes of Wales (vol i., pp. 184, 187, 189), are that "three lengths of a barleycorn is the inch". 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 "[a]s 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. by 1843] kept in the Exchequer chamber,
Guildhall, and
that was the legal definition of the inch. 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 ordered the physical realisation 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
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, effectively endorsing Johansson's pragmatic choice of conversion ratio. the United States on 1 July 1959; Australia in 1961, effective 1 January 1964; and the United Kingdom in 1963, effective on 1 January 1964. The new standards gave an inch of exactly 25.4 mm, 1.7 millionths of an inch longer than the old imperial inch and 2 millionths of an inch shorter than the old US inch. ==Related units==