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For the units of pressure, see: inch of water, inch of mercury
A unit of length in the English-speaking world = 2.54 centimeters exactly, in the United States since July 1, 1959 (see international yard), although that value had been adopted as industrial practice by the American Standards Assn. in 1933.
Symbol, the mark ″, or “in”. In the 18th century, the symbol was “n”.
Legally the inch is defined as ¹⁄₃₆th of a yard. Traditionally it had two additional definitions:
The inch is an old unit, used by the Saxons at least as early as the 7th century, since a law of around 602 requires malefactors to give those they have stabbed one shilling for each inch of wound. The word comes from the Latin uncia, from which we also get “ounce.” “Uncia” was the Roman name both for a twelfth of a libra (similar to a pound) and for a twelfth of a pes (similar to a foot). The division of the foot into 12 inches, and of the troy pound into 12 ounces, is a legacy from England's Roman invaders.
In 1921, responding to a proposal by the Netherlands that a conversion factor between inch and millimeters be agreed upon internationally, the American Standards Association (ASA) formed a special committee headed by the Director of the U. S. Bureau of Standards. The committee promptly recommended 25.4 mm, but no action was taken. Action was taken ten years later, probably due to proselytizing by C. E. Johanson, the manufacturer of the gage blocks commonly called “Jo blocks.” His company had been acquired by the Ford Motor Company, and in 1932 Ford recommended to the ASA that it formally adopt the 1 inch = 25.4 millimeters equivalence. Johanson himself toured the national standards laboratories in Europe to win their endorsement of the proposal. A special committee was formed, a draft proposal was prepared and presented to a general conference (with demonstrations by Johanson), various industries were polled, and on March 13, 1933, the ASA Standards Committee approved American Standard Recommended Practice for Inch-Millimeter Conversion for Industrial Use, setting 1 inch = 25.4 millimeters exactly.
The U.S. surveyor’s inch is ¹⁄₁₂th of a U.S. survey foot.
Various sources describe a Scottish inch. The inch in Scotland was and is identical with the English inch. The idea that the Scots had a slightly longer inch arose from an 18th century error (See Connor and Simpson).
1. 10 Anne chap. 16 §4.
Statutes at Large, volume IV.
Pages 509 – 510.
On rulers, yardsticks and tape measures the inch is usually divided into 16 or 32 parts, but sometimes into 10. However, various trades, such as shoemaking, have had their own, different, customs.
A law of Æthelbirht, King of Kent (reigned ~560-616)
67. If a thigh be pierced through, for each stab 6 shillings; if (the wound be) above an inch, a shilling; for two inches, 2; above three, 3 shillings.
Ancient Laws and Institutes of England…
[London : G. Eyre and A. Spottiswoode, printers to the Queen's Most Excellent Majesty], 1840.
The only surviving copy is a 12th-century manuscript, the Textus Roffensis. The numbering is a later editorial addition.
The lengthe of an ynche after some mennes opynyon, is made by the length of the barlycornes, which rule is not at all times trewe. For the lengthe of a barlycorne of some tillage is longer, and some shorter, after the fatnes and leanesse of the lande...Therfore in makynge an ynche after this rule It shulde be somtymes longer, and somtymes shorter ... the which shulde make great dyfference in mesurynge. Therfore ye shall take the length of an ynche moste truely upon an artificers rule, made of two foote in length, after the standarde of London, the which rule doth conteyne xxiiij ynches in length.
Richard de Benese.
This boke sheweth the maner of measurynge of all maner of lande.
The boke of measurying of lande: as well of woodland as plowland, & pasture in the feelde: & to compt the true nombre of acres of the same.
London: Thomas Coldwell, 1565.
Note, That Glasiers usually take their dimensions to a Quarter of an Inch; and in multiplying Feet, Inches, and Parts, the Inch is divided into 12 parts, as the Foot is, and each Part subdivided into 12, &c.
The Complete Measurer: or, The Whole Art of Measuring. 14th edition.
London: Printed for J. and F. Rivington, L. Hawes and W. Clarke and R. Collins, etc. 1775.
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Last revised: 5 October 2014.