An inch (plural: inches; symbol or abbreviation: in or, sometimes, ″ – a double prime) is the name of a unit of length in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system. There are 36 inches in a yard and 12 inches in a foot. A corresponding unit of area is the square inch and a corresponding unit of volume is the cubic inch. The inch is usually the universal unit of measurement in the United States, and is very commonly used in Canada and the United Kingdom.
The international standard symbol for inch is in (see ISO 31-1, Annex A). In some cases, 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. The two parts are then separated by a dash. For example, 6 feet 2 inches is denoted by 6′-2″.
1 international inch is equal to:
In some other languages, the word for "inch" is similar to or the same as the word for "thumb"; for example, French: pouce inch/thumb; Italian: pollice inch/thumb; Spanish: pulgada inch, pulgar thumb; Portuguese: polegada inch, polegar thumb; Swedish: tum inch, tumme thumb; Dutch: duim inch/thumb; Sanskrit: Angulam inch, Anguli Finger.
Given the etymology of the word "inch", it would seem that the inch is a unit derived from the foot, but this was probably only so in Latin and in Roman times. In English, there are records of fairly precise definitions for the size of an inch (whereas the definitions for the size of a foot are probably anecdotal), so it seems that the foot was then defined as 12 times this length. For example, the old English ynche was defined (by King David I of Scotland in about 1150) 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. To account for the much larger length later called an inch, there are also attempts to link it to the distance between the tip of the thumb and the first joint of the thumb, but this may be speculation.
There are records of the unit being used circa AD 1000 (both Laws of Æthelbert and Laws of Ælfred). An Anglo-Saxon unit of length was the barleycorn. After 1066, 3 barleycorn was equal to 1 inch; it is not clear which unit was the base unit and which the derived unit.
One source says that the inch was at one time defined in terms of the yard, itself supposedly defined as the distance between Henry I of England's nose and his thumb. This is unlikely as Henry was born in 1068.
Prior to the adoption of the international inch (see above), the United Kingdom and other countries of the Commonwealth of Nations defined the inch in terms of the Imperial Standard Yard. The United States and Canada each had their own, different, definition of the inch, defined in terms of metric units. The Canadian inch was defined to be equal to 25.4 millimetres.
In Sweden, between 1855 to 1863, the existing Swedish "working inch" of ~24.74 mm was replaced by a "decimal inch" of ~29.69 mm which was one tenth of the Swedish foot. Proponents argued that a decimal system simplifies calculations. However, having two different Swedish inch measures (and the English inch on top of that) proved to be complicated. Between 1878 to 1889 it was agreed to introduce the metric units.
However, the decimal inch survived in some building construction trades, and decimal fractions (tenths, hundredths, thousandths) of the foot are still used in land surveying.