http://en.wikipedia.org/wiki/Thou_%28length%29
"A thou (pronounced /'?a?/, also known as a mil or point, is a unit of length equal to 0.001 inch (a "milli-inch", one thousandth of an inch). It is sometimes used in engineering and in the specification of:
* the thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers; and
* manufacturing dimensions and tolerances.
* in the manufacture of automobile engines [A typical example is the thickness of the head gasket, or the ammount of material to be removed from the head to adjust the compression ratio of the cylinders.]
In the United States, as the metric system became more common thou began to replace mil among technical users due to the possible confusion with millimeters.[1][2][3] However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags."