A thousandth of an inch is a derived unit of length in a system of units using inches. Equal to 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" ( 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: The thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers For example, most plastic ID cards are about in thickness. Card stock thickness in the United States, where mils are also called points. Gauge (diameter) of strings in stringed instruments Manufacturing dimensions and tolerances, such as: In the manufacture of older automobile engines. A typical example is the thickness of the head gasket, or the amount of material to be removed from the head to adjust the compression ratio of the cylinders. In the servicing of older automobile engines. Typical examples include a spark-plug gap or ignition points gap. The manufacture of printed circuit boards (PCBs). However, the component dimensions are now typically provided in millimetres, because they are sold worldwide. Tolerance specifications on hydraulic cylinders There are also compound units such as "mils per year" used to express corrosion rates. A related measurement for area known as the circular mil, is based on a circle having a diameter of one mil. Tenths In machining, where the thou is often treated as a basic unit, can be referred to as "one tenth", meaning "one tenth of a thou" or "one ten thousandth". (The metric comparison is discussed below.) Machining "to within a few tenths" is often considered very accurate, and at or near the extreme limit of tolerance capability in most contexts. Greater accuracy (tolerance ranges inside one tenth) apply in only a few contexts: in plug gauge and gauge block manufacturing or calibration, they are typically expressed in millionths of an inch or, alternatively, in micrometres; in nanotechnology, nanometres or picometres are used. Usage notes on mil