Thousandth of an inch different from international 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: