An inch (plural: inches; abbreviation or symbol: in or ″ – a double prime) is a unit of length in the imperial and United States customary systems of measurement.Historically, an inch was also used in a number of other systems of units.
It is also used in Japan for electronic parts, especially 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) 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. (This is akin to how the first and second "cuts" of the hour are likewise indicated by prime and double prime symbols.) Subdivisions of an inch are typically written using dyadic fractions with odd number numerators; for example, The English word inch comes from Latin uncia meaning "one-twelfth part" (in this case, one twelfth of a foot); the word ounce (one twelfth of a troy pound) has the same origin.
The vowel change from u to i is umlaut; the consonant change from c (pronounced as k) to ch is palatalisation (see Old English phonology).
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); for example, Catalan: Paragraph LXVII sets out the fine for wounds of various depths: one inch, one shilling, two inches, two shillings, etc. ofer þry, iii scill." An Anglo-Saxon unit of length was the barleycorn.
"Gif man þeoh þurhstingð, stice ghwilve vi scillingas. 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One, dating from the first half of the 10th century, is contained in the Laws of Hywel Dda which superseded those of Dyfnwal, an even earlier definition of the inch in Wales.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".