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millimeter, mm

Abbreviated mm, no period.

In text, usually spelled out as a noun or adjective, although may be abbreviated as an adjective in some
standard uses:
            a caterpillar 25 millimeters long
            a ten-millimeter-wide line
but     16-mm film
            7.7-mm shell

In photographic data, film and lens sizes are often written without a hyphen:
            35mm film
            200mm zoom

Either style is accepted as long as it is consistent.

See also kilometer, km, meter, m, centimeter, cm, METRIC AND INTERNATIONAL MEASUREMENT.