M stands for 'thousand':

https://en.wikipedia.org/wiki/Roman_numerals

I don't think people use Roman numerals when they say 500k to mean 500 thousand. It is implied the SI system is used. How does it switch from SI to Roman numerals when talking about millions? Also MM is 2000.

MM -> 1000 x 1000

It's inconsistent but since it is in common use I wouldn't fret about it and just accept the fact.

I didn't downvote you, but MM in Roman numerals is million. It is also common enough to represent roman numerals in lowercase that mm, as used here, is totally valid.

Actually, MM in Roman numerals is 2,000, not a million.

You’re right. Don’t know what I was thinking. Guess it’s an accounting only thing

Million is M, not mm or MM or M&M

I believe some parts of the world, particularly in Europe, interpret $500M as $500 * 10^3 (not $500 * 10^6)...So some use $500MM to universally avoid this confusion

mm and MM are very common ways express "million" in finance. M, mm, MM all mean the same thing.

Thousand is M.

In SI, M is million same as k is a thousand.