A thou (pronounced /ˈθaʊ/, 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). The micrometre is the SI unit most similar in size.

The introduction of the thou in 1844 is generally attributed to Joseph Whitworth.^[1]

[edit] Usage

The mil or thou 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 amount of material to be removed from the head to adjust the compression ratio of the cylinders.]
• in the servicing of automobile engines [Typical examples include a spark-plug gap or ignition points gap]

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.^[2][3][4] However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags.

In the United States, the mil/thou is still in use extensively in certain industries such as in the manufacture of printed circuit boards (PCBs) and for tolerance specifications on hydraulic cylinders.

A "mil" is equal to one thousandth of an inch. So, 30 mils, most common thickness of most plastic ID cards, is equal to 0.03 inches (0.76 mm).

The plural of thou is also thou; e.g., a measurement of one hundredth of an inch is described as 10 thou. The th in thou is pronounced as in thousand, /ˈθaʊ/, unlike the pronoun thou, /ðaʊ/, where the th is pronounced as in that. The plural of mil is mils.

There is a related measurement for area known as the circular mil, based on a circle having a diameter of one mil. Because of this relationship, a mil and a circular mil might be easily confused.

[edit] Equivalence to other units of length

1 thou is exactly equal to:

• 0.001 international inches (1 international inch is equal to 1,000 thou)
• 0.0254 mm, or 25.4 μm (1 millimeter is about equal to 39.37 thou)

[edit] Notes