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Measuring tape with inches The inch (symbol: in or ) is a
unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * ''Unit'' (a ...
of length in the
British imperial The British Empire was composed of the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts esta ...
and the United States customary
systems of measurement A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement i ...
. It is equal to
yard The yard (symbol: yd) is an English unit of length in both the British imperial and US customary systems of measurement equalling 3  feet or 36 inches. Since 1959 it has been by international agreement standardized as exactly ...
or of a foot. Derived from the Roman uncia ("twelfth"), the word ''inch'' is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb. Standards for the exact length of an inch have varied in the past, but since the adoption of the
international yard The international yard and pound are two units of measurement that were the subject of an agreement among representatives of six nations signed on 1 July 1959: the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. T ...
during the 1950s and 1960s the inch has been based on the
metric system The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Interna ...
and defined as exactly 25.4 mm.


Name

The English word "inch" ( ang, ynce) was an early borrowing from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
' ("one-twelfth; Roman inch;
Roman ounce The (plural: ) was a Roman unit of length, weight, and volume. It survived as the Byzantine liquid ounce ( grc-gre, οὐγγία, ''oungía'') and the origin of the English inch, ounce, and fluid ounce. The Roman inch was equal to of a Ro ...
"). The vowel change from Latin to Old English (which became Modern English ) is known as umlaut. The consonant change from the Latin (spelled ''c'') to English is palatalisation. Both were features of Old English phonology; see and for more information. "Inch" is cognate with "
ounce The ounce () is any of several different units of mass, weight or volume and is derived almost unchanged from the , an Ancient Roman unit of measurement. The avoirdupois ounce (exactly ) is avoirdupois pound; this is the United States customa ...
" ( ang, ynse), whose separate pronunciation and spelling reflect its reborrowing in
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old 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). In the
Dutch language Dutch ( ) is a West Germanic language spoken by about 25 million people as a first language and 5 million as a second language. It is the third most widely spoken Germanic language The Germanic languages are a branch of the Indo-Europea ...
a term for inch is ''engelse duim'' (english thumb). Examples include ca, polzada ("inch") and ' ("thumb"); cs, palec ("thumb"); Danish and no, tomme ("inch") ' ("thumb"); nl, duim (whence af, duim and russian: дюйм); french: pouce; hu, hüvelyk; it, pollice; pt, polegada ("inch") and ' ("thumb"); ("duim"); sk, palec ("thumb"); es, pulgada ("inch") and ' ("thumb"); and sv, 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 in
tyre code Automotive tires are described by an alphanumeric tire code (in North American English) or tyre code (in Commonwealth English), which is generally molded into the sidewall of the tire. This code specifies the dimensions of the tire, and some o ...
s. The international standard symbol for inch is in (see
ISO 31-1 ISO 31-1 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to ''space and time''. It was superseded in 2006 by ISO 80000-3. Definitions Its definitions include: Annex A Annex ...
, Annex A) but traditionally the inch is denoted by a double prime, which is often approximated by a double quote symbol, and the foot by a
prime A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...
, which is often approximated by an apostrophe. For example; ' can be written as 3′ 2″. (This is akin to how the
first First or 1st is the ordinal form of the number one (#1). First or 1st may also refer to: *World record, specifically the first instance of a particular achievement Arts and media Music * 1$T, American rapper, singer-songwriter, DJ, and rec ...
and second "cuts" of the hour are likewise indicated by prime and double prime symbols, and also the
first First or 1st is the ordinal form of the number one (#1). First or 1st may also refer to: *World record, specifically the first instance of a particular achievement Arts and media Music * 1$T, American rapper, singer-songwriter, DJ, and rec ...
and second cuts of the degree.) Subdivisions of an inch are typically written using dyadic fractions with odd number numerators; for example, ' would be written as ″ and not as 2.375″ nor as ″. However, for engineering purposes fractions are commonly given to three or four places of decimals and have been for many years.


Equivalents

1 international inch is equal to: * 10,000 ' tenths' * 1,000
thou The word ''thou'' is a second-person singular pronoun in English. It is now largely archaic, having been replaced in most contexts by the word '' you'', although it remains in use in parts of Northern England and in Scots (). ''Thou'' is the ...
or mil * 100 points or gries * 72 PostScript points * 10, 12, or 40
lines Line most often refers to: * Line (geometry), object with zero thickness and curvature that stretches to infinity * Telephone line, a single-user circuit on a telephone communication system Line, lines, The Line, or LINE may also refer to: Arts ...
* 6 computer picas * 3 barleycorns * 25.4 millimetres exactly (1 millimetre ≈ 0.03937008 inches) * 0.999998 US Survey inches * or 0.33 palms * or 0.25 hands * or 0.0833
feet The foot ( : feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made ...
* or 0.0277
yard The yard (symbol: yd) is an English unit of length in both the British imperial and US customary systems of measurement equalling 3  feet or 36 inches. Since 1959 it has been by international agreement standardized as exactly ...
s


History

The earliest known reference to the inch in England is from the '' Laws of Æthelberht'' dating to the early 7th century, surviving in a single manuscript, the ''
Textus Roffensis __NOTOC__ The ''Textus Roffensis'' (Latin for "The Tome of Rochester"), fully titled the ''Textus de Ecclesia Roffensi per Ernulphum episcopum'' ("The Tome of the Church of Rochester up to Bishop Ernulf") and sometimes also known as the Anna ...
'' from 1120. Paragraph LXVII sets out the fine for wounds of various depths: one inch, one shilling; two inches, two shillings, etc. Translation (taken from ): If a thigh is pierced right through, 6 shillings compensation shall be paid for each stab. For a stab over an inch eep 1 shilling; for a stab between 2 and 3 inches, 2 shillings; for a stab over 3 inches 3 shillings. An Anglo-Saxon unit of length was the barleycorn. 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 Edward II (25 April 1284 – 21 September 1327), also called Edward of Caernarfon, was King of England and Lord of Ireland from 1307 until he was deposed in January 1327. The fourth son of Edward I, Edward became the heir apparent to t ...
, defining it as "three grains of
barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley p ...
, dry and round, placed end to end, lengthwise". Similar definitions are recorded in both English and Welsh medieval law tracts. One, dating from the first half of the 10th century, is contained in the Laws of
Hywel Dda Hywel Dda, sometimes anglicised as Howel the Good, or Hywel ap Cadell (died 949/950) was a king of Deheubarth who eventually came to rule most of Wales. He became the sole king of Seisyllwg in 920 and shortly thereafter established Deheubart ...
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". King David I of Scotland in his Assize of Weights and Measures (c. 1150) is said to have defined the Scottish inch as the width of an average man's thumb at the base of the nail, even including the requirement to calculate the average of a small, a medium, and a large man's measures. However, the oldest surviving manuscripts date from the early 14th century and appear to have been altered with the inclusion of newer material. In 1814, Charles Butler, a mathematics teacher at Cheam School, recorded the old legal definition of the inch to be "three grains of sound ripe barley being taken out the middle of the ear, well dried, and laid end to end in a row", and placed the barleycorn, not the inch, as the base unit of the English Long Measure system, from which all other units were derived.
John Bouvier John Bouvier (1787 – November 18, 1851), was a French-American jurist and legal lexicographer, is known for his legal writings, particularly his ''Law Dictionary Adapted to the Constitution and Laws of the United States of America and of the S ...
similarly recorded in his 1843 law dictionary that the barleycorn was the fundamental measure. Butler observed, however, that " the length of the barley-corn cannot be fixed, so the inch according to this method will be uncertain", noting that a standard inch measure was now .e. by 1843kept in the Exchequer chamber,
Guildhall A guildhall, also known as a "guild hall" or "guild house", is a historical building originally used for tax collecting by municipalities or merchants in Great Britain and the Low Countries. These buildings commonly become town halls and in som ...
, and ''that'' was the legal definition of the inch. This was a point also made by George Long in his 1842 '' Penny Cyclopædia'', observing that standard measures had since surpassed the barleycorn definition of the inch, and that to recover the inch measure from its original definition, in case the standard measure were destroyed, would involve the measurement of large numbers of barleycorns and taking their average lengths. He noted that this process would not perfectly recover the standard, since it might introduce errors of anywhere between one hundredth and one tenth of an inch in the definition of a yard. Before the adoption of the
international yard and pound The international yard and pound are two units of measurement that were the subject of an agreement among representatives of six nations signed on 1 July 1959: the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. T ...
, various definitions were in use. In the United Kingdom and most countries of the
British Commonwealth The Commonwealth of Nations, simply referred to as the Commonwealth, is a political association of 56 member states, the vast majority of which are former territories of the British Empire. The chief institutions of the organisation are the Co ...
, the inch was defined in terms of the Imperial Standard Yard. The United States adopted the conversion factor 1 metre = 39.37 inches by an act in 1866. In 1893, Mendenhall ordered the physical realization of the inch to be based on the international prototype metres numbers 21 and 27, which had been received from the CGPM, together with the previously adopted conversion factor. As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977 mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing
gauge blocks Gauge blocks (also known as gage blocks, Johansson gauges, slip gauges, or Jo blocks) are a system for producing precision lengths. The individual gauge block is a metal or ceramic block that has been precision ground and lapped to a specific ...
in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the ''de facto'' standard for manufacturers internationally, with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his. In 1930, the
British Standards Institution The British Standards Institution (BSI) is the national standards body of the United Kingdom. BSI produces technical standards on a wide range of products and services and also supplies certification and standards-related services to busines ...
adopted an inch of exactly 25.4 mm. The American Standards Association followed suit in 1933. By 1935, industry in 16 countries had adopted the "industrial inch" as it came to be known, effectively endorsing Johansson's pragmatic choice of conversion ratio. In 1946, the Commonwealth Science Congress recommended a yard of exactly 0.9144 metres for adoption throughout the British Commonwealth. This was adopted by Canada in 1951; the United States on 1 July 1959; Australia in 1961, effective 1 January 1964; and the United Kingdom in 1963, effective on 1 January 1964. The new standards gave an inch of exactly 25.4 mm, 1.7 millionths of an inch longer than the old imperial inch and 2 millionths of an inch shorter than the old US inch.


Related units


US survey inches

The United States retains the -metre definition for surveying, producing a 2 millionth part difference between standard and US survey inches.A. V. Astin & H. Arnold Karo, (1959)
''Refinement of values for the yard and the pound''
Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59-5442, Filed, 30 June 1959, 8:45 am)
This is approximately  inch per mile; 12.7 kilometres is exactly standard inches and exactly survey inches. This difference is substantial when doing calculations in State Plane Coordinate Systems with coordinate values in the hundreds of thousands or millions of feet. In 2020, the U.S. NIST announced that the U.S. survey foot would "be phased out" on 1 January 2023 and be superseded by the International foot (also known as the foot) equal to 0.3048 metres exactly, for all further applications. and by implication, the survey inch with it.


Continental inches

Before the adoption of the metric system, several European countries had customary units whose name translates into "inch". The French '' pouce'' measured roughly 27.0 mm, at least when applied to describe the calibre of artillery pieces. The Amsterdam foot (''voet'') consisted of 11 Amsterdam inches (''duim''). The Amsterdam foot is about 8% shorter than an English foot.


Scottish inch

The now obsolete Scottish inch ( gd, òirleach), of a Scottish foot, was about 1.0016 imperial inches (about ).


See also

*
English units English units are the units of measurement used in England up to 1826 (when they were replaced by Imperial units), which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at ...
*
Square inch A square inch (plural: square inches) is a unit of area, equal to the area of a square with sides of one inch. The following symbols are used to denote square inches: *square in *sq inches, sq inch, sq in *inches/-2, inch/-2, in/-2 *inches^2, inc ...
,
Cubic inch The cubic inch (symbol in3) is a unit of volume in the Imperial units and United States customary units systems. It is the volume of a cube with each of its three dimensions (length, width, and height) being one inch long which is equivalent ...
, and Metric inch *
International yard and pound The international yard and pound are two units of measurement that were the subject of an agreement among representatives of six nations signed on 1 July 1959: the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa. T ...
*
Anthropic units The term anthropic unit (from Greek '' άνθρωπος'' meaning human) is used with different meanings in archaeology, in measurement and in social studies. In archaeology In archaeology, ''anthropic units'' are strata, or deposits of materi ...
* Pyramid inch * Digit and Line


Notes


References


Citations


Bibliography

* {{Authority control Customary units of measurement in the United States Imperial units Units of length Obsolete Scottish units of measurement fy:Tomme (lingtemaat)