INCH

The inch (symbol: in or ) is a unit of length in the British imperial and the United States

customary systems of measurement. It is equal to 1/36 yard or 1/12 of a foot. Derived from the Roman

uncia (“twelfth”), the word inch is also sometimes used to translate similar units in other measurements

systems usually understood as deriving from the width of the human thumb

The English word “inch” (Old English: ynce) was an early borrowing from Latin uncia (“one-twelfth; Roman inch; Roman ounce”). The vowel change from Latin /u/ to Old English /y/ (which became Modern English /ɪ/) is known as umlaut. The consonant change from the Latin /k/ (spelled c) to English /tʃ/ is palatalization. Both were features of Old English phonology; see Phonological history of Old English § Palatalization and Germanic umlaut § I-mutation in Old English for more information.

Name

“Inch” is cognate with “ounce” (Old English: NYSE), 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). Examples include Catalan: polzada (“inch”) and polze (“thumb”); Czech: palec (“thumb”); Danish and Norwegian: tomme (“inch”) tommel (“thumb”); Dutch: duim (whence Afrikaans: duim and Russian: дюйм); French: pouce; Hungarian: hüvelyk; Italian: pollice; Portuguese: polegada (“inch”) and polegar (“thumb”); (“duim”); Slovak: palec (“thumb”); Spanish: pulgada (“inch”) and pulgar (“thumb”); and Swedish: tum (“inch”) and tumme (“thumb”).

Usage

The inch is a commonly used customary unit of length in the United States, Canada, and the United Kingdom. It is also used in Japan for electronic parts, especially display screens. In most of continental Europe, the inch is also used informally as a measure for display screens. For the United Kingdom, guidance on public sector use states that, since 1 October 1995, without time limit, the inch (along with the foot) is to be used as a primary unit for road signs and related measurements of distance (with the possible exception of clearance heights and widths) and may continue to be used as a secondary or supplementary indication following a metric measurement for other purposes.

Inches are commonly used to specify the diameter of vehicle wheel rims, and the corresponding inner diameter of tyres – the number following the letter R in a car/truck tyre size such as 235/75R16; The first three-digit number refers to the tyre’s width (normally expressed in millimetres). The following two-digit number indicates the ratio of the tyre’s height to its width (in this example: the height is 75% of the width). The letter “R” stands for Radial, which means the layers run radially across the tyre. The number following the R specifies the diameter of the wheel, normally expressed in inches (in this example, the diameter of the wheel is 16 inches).

The international standard symbol for inch is in (see ISO 31-1, Annex A) but traditionally the inch is denoted by a double prime, which is often approximated by double quotes, and the foot by a prime, which is often approximated by an apostrophe. For example; three feet, two inches can be written as 3′ 2″. (This is akin to how the first and second “cuts” of the hour are likewise indicated by prime and double prime symbols, and also the first and second cuts of the degree.)

Subdivisions of an inch are typically written using dyadic fractions with odd number numerators; for example, two and three-eighths of an inch would be written as 2+3/8″ and not as 2.375″ nor as 2+6/16″. However, for engineering purposes fractions are commonly given to three or four places of decimals and have been for many years.

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