[jahewson]

Millimetre is mm, not MM which would be "meter meter" which is nonsense. MM is actually the roman numeral for 1 million.

[anonymouz]

Capital M would be 'mega' as a prefix, but I think it does not exist as a unit. If you're willing to read MM as Mm, then it would be Megameter.

Your really should revisit roman numerals. MM = 2000, you have to add them, not multiply.

[jahewson]

Good point. So it turns out that MM is supposed to mean "thousand thousand" in the world of finance, but it is indeed not a correct roman numeral. Old school.