Mid-19th century tool for converting between different standards of the inch
An inch is an Imperial 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 the word for "thumb" in some languages. French: pouce inch, pouce thumb; Italian: pollice inch, pollice thumb; Spanish: pulgada inch, pulgar thumb; Swedish: tum inch, tumme thumb. In Dutch it is even the same: duim inch and thumb.
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. Today there are two units called the "inch" still in use, both being largely confined to the United States. Other countries, which previously had their own separate definitions of the inch, have converted to using the metric system instead. When the inch being referred to is not specified, it almost always means the international inch.
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 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 copy, and hence Soviet parts didn't match exactly with Western ones.
U.S. survey foot
However, the US continued to use its previous national definition of the length units for surveying purposes. The US survey foot is defined so that 1 metre is exactly 39.37 inches (inches, however, are rarely if ever used in surveying). The international inch is exactly 0.999998 times the old US definition of an inch; 1 survey inch equals 25.4/0.999998 or approximately 25.4000508001016002032004(...) mm. Whilst the difference between the two units is only two parts per million, the difference between the two units makes a significant difference of many metres when the unit is used to define measurements made on the scale of distances of thousands of kilometres.
As a result the U.S. survey acre is 4046.8726 m², compared with the international acre which is 4046.8564 m², a difference equal to about a sheet of A6 paper or 1/4 of a U.S. letter size paper.
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 — and not as in "that" or the pronoun "thou" — IPA /ðaʊ/.
The inch unit may be denoted by a double prime (ex. 30″ = 30 inches), often approximated by a quotation mark. Similarly, feet can be denoted by a prime (often approximated by an apostrophe), so 6′2″ means 6 feet 2 inches.
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 1/10th 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.