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An inch is an Imperial and U.S. customary unit of length. Sweden also briefly had a "decimal inch" based on the metric system: see below for more.
According to some sources, the inch was originally defined informally as the distance between the tip of the thumb and the first joint of the thumb. Another source says that the inch was at one time defined in terms of the yard, supposedly defined as the distance between Henry I of England's nose and his thumb. There are twelve inches in a foot, and three feet in a yard.
The word for "inch" is similar to or the same as the word for "thumb" in some languages. French: pouce inch, pouce thumb; Italian: pollice inch, pollice thumb; Spanish: pulgada inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb; Dutch: duim inch, duim thumb.
International inch
Template:Unit of length Historically, the inch has referred to several slightly different units of length, used in different parts of the world. There was little uniformity; different countries, and even different cities within the same country, used their own standard length. The only "inch" still in use today is the English unit. Other countries, which previously had their own separate definitions of the inch, have converted to using the metric system instead. However, the inch remains a common and popular unit of measurement in the United Kingdom and in Canada, along with the mile, pound and others, despite official attempts to eradicate it. In the UK personal heights are expressed in feet and inches by people of all ages (except in official usage), but usage of the inch for other purposes is not frequent among younger people who prefer to use centimetres although use of the inch does increase due to the inch's general use.
The international inch is defined in terms of the metric system of units to be exactly 25.4 mm. This definition was agreed upon by the U.S. and countries of the British Commonwealth in 1958. Prior to that, the U.S. and Canada each had their own, slightly different, definition of the inch in terms of metric units, while the UK and other Commonwealth countries defined the inch in terms of the Imperial Standard Yard. The definition adopted was the Canadian definition. A metric inch was also used in some Soviet clones of Western computers. The clones were a slightly scaled-down copy, and hence Soviet parts did not match exactly with Western ones.
The inch is used in Japan for various purposes, such as the sizes of bicycle tyres or television screens.
The thou
The thou or mil is a unit sometimes used in engineering equivalent to one-thousandth of an international inch, and thus defined to be 25.4 µm. Use of the thou is now generally deprecated in favour of the use of SI units. When "thou" is the measurement, its "th" is pronounced as in "thousand" — IPA /θaʊ/ — and not as in "that" or the pronoun "thou" — IPA /ðaʊ/.
Notation
The international standard symbol for inch is in (see ISO 31-1, Annex A). In some areas, the unit inch is also denoted by a double prime (ex. 30″ = 30 in), often approximated by a quotation mark. Similarly, feet can be denoted by a prime (often approximated by an apostrophe), and then 6′2″ means 6 feet plus 2 inches. However, since the prime and double prime are the international standard symbols for arcminutes and arcseconds, this can, in certain contexts, cause confusion.
Sweden
In the 19th century, Sweden devised a way into the metric world. First, in 1855–1863 the existing "working inch" was changed into a "decimal inch" which was 1/10 foot or approximately 0.03 metres. Proponents argued that a decimal system simplifies calculations, but having two different inch measures turned out to be so complicated that in 1878–1889 it was agreed to introduce the metric units.