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, the international inch and the U.S. survey inch, 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 which inch being referred to is not specified, it almost always means the international inch.
The international inch is defined as exactly 25.4 mm. This definition was agreed upon by the U.S. and the U.K. in 1958. Prior to that, each country had its own, slightly different definition of the inch in terms of metric units.
However, the U.S. continued to use its previous national definition of the inch for surveying purposes. This 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.
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 1/10 foot or approximately 0.03 meters. The quite reasonable motivation was that a decimal system simplifies calculations, but having two different inch measures turned out to be so complicated that in 1878-1889 everybody agreed to introduce the metric units.
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