Tl;dr - It's historical usage to disambiguate in finance.

http://english.stackexchange.com/a/182072

In my experience when you see "MM" they have some kind of finance background, probably some time in banking. They could be aping someone else of course.

It's just an old-school convention. I always write it like that myself, and it looks weird to me when there is only a single lower-case "m".

heh, I guess I just never have needed to write about millions of dollars on official documents.

'M' is the roman numeral for thousand, so 25m traditionally means 25000. mm is thousand thousand, aka million.

And SI uses 'M' as a prefix for one million. Oxford defines 'm' as million(s) in a money context, Merriam-Webster includes 'M' as an abbreviation of million.

It seems odd that anyone would think $25m should be read as $25000. It's much more likely that I would read $25mm as 25 millimeters instead.

The problem is that there are multiple traditional usages, with overlapping meanings.

Capital M has traditionally stood for both thousand and million. In other words $20M and $20m is ambiguous, depending on what tradition you come from. $20MM and $20MM and $20k are not ambiguous.

That notation doesn't really make much sense though, since MM is roman for two thousand.

Yeah and also literally no-one says "25M" when they mean 25000.

I believe the traditional usage requires capital M:

   $20M = $20,000

   $6MM = $6,000,000

Of course, it's more common now to use lowercase k:

   $30k = $30,000

25MD (Megadollars) would be nice. Similarly, why don't we ever talk about Megameters?

A, uh, personal eccentricity of mine is to treat dollars like any other unit written after the amount and optionally taking SI prefixes (At least in any writing I can get away with it). It's doubly nice because it plays well with rates/other composite units.

So:

$1,000 -> 1 k$

$1 million -> 1 M$

$1 billion -> 1 G$

$100,000 per year -> 100 k$/yr

I'm sure it'll never catch on broadly, but it's conceptually pleasing to me.

Just as long as you don't write it "$1,000 dollars".

I mean, is that 1,000 $^2?

> Similarly, why don't we ever talk about Megameters?

I think we do - I went on a 10 megametre trip when I went to Japan by land and sea. It's just very rarely a convenient unit because not many things are that long.

I find myself describing distances as thousands of kilometers fairly often.

But even then, I'll admit thousands of km is easier to immediately picture than Mm. "Change your oil every 15Mm" would look odd in an owner's manual.