Thousandth of an inch different from US survey mil
A thousandth of an inch is a derived unit of length in a system of units using inches. Equal to 1⁄1000 of an inch, a thousandth is commonly called a thou  (used for both singular and plural) or particularly in North America a mil (plural mils).
The words are shortened forms of the English and Latin words for "thousand" (mille in Latin).  In international engineering contexts, confusion can arise because mil is a formal unit name in North America but mil or mill is also a common colloquial clipped form of millimetre. The units are considerably different: a millimetre is approximately 39 mils.Contexts of use
The thou, or mil, is most commonly used in engineering and manufacturing in non-metric countries. For example, in specifying:Usage notes on mil versus thou
In the United States, mil was once the more common term, but as use of the metric system has become more common, thou has replaced mil among most technical users to avoid confusion with millimetres. Today both terms are used, but in specific contexts one is traditionally preferred over the other.