Thou (unit of length)

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Thou (unit of length)

A thou, also known as a mil or point, is a unit

Units of measurement

The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day....
of length

Length

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....
equal to 0.001 inch

Inch

An inch is the name of a Units of measurement of length in a number of different systems, including Imperial units, and United States customary units....
(a "milli-inch" or a "thousandth of one inch"). It is sometimes used in engineering and in the specification of:

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. However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags.

Internationally, use of the thou is now generally deprecated in favour of the use of SI

Si, si, or SI may refer to :...
units of length such as the micrometre

Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
.

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Encyclopedia

A thou, also known as a mil or point, is a unit

Units of measurement

The definition, agreement and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day....
of length

Length

Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end....
equal to 0.001 inch

Inch

the thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers; and
manufacturing dimensions and tolerances
Tolerance (engineering)

Engineering tolerance is the permissible limit of variation in# a physical dimension,# a measured value or physical property of a material, manufacturing object, system, or service,...
.
in the manufacture of certain racing engines [Normally the space between the valves and pistons, or camshaft and valves]

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. However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags.

Internationally, use of the thou is now generally deprecated in favour of the use of SI

Si, si, or SI may refer to :...
units of length such as the micrometre

Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
. In the United States, the mil/thou is still in use extensively in certain industries such as in the manufacture of printed circuit boards

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using Conductor pathways, or signal traces, industrial etchinged from copper sheets laminated onto a non-conductive substrate....
(PCBs) and for tolerance specifications on hydraulic cylinders

Hydraulic cylinder

A Hydraulic cylinder is a mechanical actuator that is used to give a linear force through a linear stroke. It has many applications, notably in engineering vehicles....
.

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, , unlike the pronoun thou
Thou

The word thou is a grammatical person grammatical number pronoun in English language. It is now largely archaism, having been replaced in almost all contexts by you....
, , where the th is pronounced as in that.

The plural of mil is mils.

There is a related measurement for area

Area

Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....
known as the circular mil

Circular mil

A circular mil is a Units of measurement of area, equal to the area of a circle with a diameter of one Thou . A mil is one thousandth of an inch....
, based on a circle

Circle

A circle is a simple shape of Euclidean geometry consisting of those point in a plane which are the same distance from a given point called the center....
having a diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle....
of one mil. Because of this relationship, a mil and a circular mil might be easily confused.

Equivalence to other units of length

1 thou is exactly equal to:

0.001 international inch
Inch

An inch is the name of a Units of measurement of length in a number of different systems, including Imperial units, and United States customary units....
es (1 international inch is equal to 1,000 thou)
0.0254 mm
Millimetre

The millimetre is a Units of measurement of length in the metric system, equal to one thousandth of a metre, which is the current International System of Units SI base unit of length....
, or 25.4 µm
Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
(1 millimetre is about equal to 39.37 thou)