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As noted in another comment; MM is 2000 in roman numerals.

Never heard of such a notation before. Probably only used in the US where people don't use mm.

MM is a million. mm is a millimetre.

In the metric system,

1 mm = 0.001 m = 0.001 metres

but

1 MM = 1000000 M = 1000000 ?????

As somebody else pointed out, MM is two thousand, not a million.

MM, correct or not, is finance lingo for million (1000*1000).

Just like 1 KB is 1024 bytes, not 1000. ;)