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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.

Historically, the inch has referred to several slightly different units of length, used in different parts of the world. 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 U.K. and other Commonwealth countries defined the inch in terms of the Imperial Standard Yard. The definition adopted was the Canadian definition.

However, the U.S. continued to use its previous national definition of the inch for surveying purposes. This inch, known as the U.S. survey inch, is defined so that 1 metre is exactly 39.37 survey inches. 1 survey inch equals approximately 25.40000508 mm, or 1.000002 international inches. Whilst the difference between the two units is only approximately two parts per million, the difference between the two units makes a significant difference of many meters when the unit is used to define measurements made on the scale of distances of thousands of kilometers.

The thou (pronounced "thow" as in thousandth, not "thou" as in the pronoun) 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.

The unit is sometimes denoted by a quotation mark (ex. 30" = 30 inches).