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 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 which cannot be defined simply in hundredths unlike (0.25) or (0.5). Traditional fractions continued to be used, as they were a substantial part of other units of length.
Usage.
The thou is used in engineering and manufacturing mostly.