inch

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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 18^th century, the symbol was “n”.

Legally the inch is defined as ¹⁄₃₆th of a yard. Traditionally it had two additional definitions:

the length of three barleycorns from the middle of the ear, placed end to end.
the width of the thumb at the base of the nail. The thumb inch was especially common in the cloth trade until 1711, when Queen Anne explicitly forbade its use.¹

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, 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 18^th century error (See Connor and Simpson).

1. 10 Anne chap. 16 § 4.
Statutes at Large, volume IV.
1786.

Pages 509 – 510.