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The thou (abbreviation th) or mil (Chiefly United States) is a unit of length in the Imperial System and the United States Customary Units. It is equal to 1⁄1,000 of an inch.
Thou comes from the Germanic Languages, a shortening of thousandth.
Mil comes from the Latin Languages, a shortening of mille.
The thou was introduced in 1844 by Joseph Whitworth as a substitute to traditional fractions such as 1⁄64 1⁄32 1⁄16 1⁄8 which cannot be defined simply in hundredths unlike 1⁄4 (0.25) or 1⁄2 (0.5). Traditional fractions continued to be used, as they were a substantial part of other units of length.
| International | Thou | Inch | Millimetre |
|---|---|---|---|
| Thou | 1 | 1⁄1,000 | 0.0254 |
The thou is used in engineering and manufacturing mostly.
