millimeters are used to measure things as an example 1000mm= 1 metre.
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a millimeter
Metre.
Depending how small it is, you could use the meter, the millimeter, the micron (= micrometer), the nanometer, etc. The meter is approximately the height of a child; the millimeter is 1/1000 of that, the micron is a millionth of a meter, the nanometer is a billionth of a meter.
Do you mean what is the abbreviation for millimeter? Because that is "mm". When you write units in fractions, you will probably want to use the abbreviation.
20 is greater than 10; therefore, 20mm would be thicker than 10mm.