Thou (unit)

The thou (abbreviation th) or mil (mainly 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. This is not to be confused with the old form of "you".

Etymology

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

Mil comes from the Romance languages, a shortening of mille.

History

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.

Length

International	Thou	Inch	Millimetre
Thou	1	​1⁄1,000	0.0254

Usage

The thou is used in engineering and manufacturing mostly.