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|SI units||×10−3 m1|
The millimetre (International spelling as used by the International Bureau of Weights and Measures) or millimeter (American spelling) (SI unit symbol mm) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length.
Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/ of a second". A millimetre, 1/ of a metre, is therefore the distance travelled by light in 1/ of a second.
On a metric ruler, the smallest measurements are normally millimetres. High-quality engineering rules may be graduated in increments of 0.5 mm. Digital Vernier callipers are commonly capable of reading increments as small as 0.01 mm.
Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths between 30 GHz and 300 GHz for data transmission, in contrast to the 300 MHz to 3 GHz normally used in mobile devices, has the potential to allow data transfer rates of 10 gigabits per second.
The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a human hair. A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately a tenth of a millimetre thick.
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