Thou (unit)

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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 11,000 of an inch.

Etymology[change | change source]

Thou comes from the Germanic Languages, a shortening of thousandth.

Mil comes from the Latin Languages, a shortening of mille.

History[change | change source]

The thou was introduced in 1844 by Joseph Whitworth as a substitute to traditional fractions such as 164 132 116 18 which cannot be defined simply in hundredths unlike 14 (0.25) or 12 (0.5). Traditional fractions continued to be used, as they were a substantial part of other units of length.

Length[change | change source]

International Thou Inch Millimetre
Thou 1 11,000 0.0254

Usage[change | change source]

The thou is used in engineering and manufacturing mostly.