Doubling to indicate a plural initialism, like "pp 3-7" for "pages 3-7" or "EE. UU." for "Estados Unidos".

Roman number M = 1 thousand

1 thousand 1 thousands = 1 million

We don't use Roman numerals for thousands, why should we do so for millions? The right way is obviously using metric prefixes, so 1k$, 1M$ and 1G$.

And don't forget that 'cents' is short for 'centidollars'.

Is there a special reason the 'k' is lowercase?

Well, M for mega is capitalized since m is already used by milli. And all of the SI prefixes larger than mega are also capital, I guess to denote that they're large...

All of the SI prefixes k and below are lowercase.

Exactly, capital M

I've always seen k used to represent one thousand in dollar amounts, so I've never interpreted M as being a Roman numeral but simply as standing for "million."

Disclaimer: I've been in the US my whole life.

But, then that would be 150 * 2000 = 300,000.

Yes. If we're talking Roman numerals, MM would be 2000, just like XX is 20 -- not 100. This notation makes no sense to me either.

Exactly. 1 million in Roman numerals is M with a bar over the top of it.

m is 1,000. mm is m * m or 1000 * 1000, which is 1,000,000

MM, sure. mm, however, is millimeters.

its done in finance sometimes and the thought behind it is m = 1000, so multiply 1000 by 1000 and you get 1,000,000.

Personally I hate that notation and use capital M whenever given the opportunity. I feel like it used to show superiority or present another barrier to people outside of finance looking in.

a millimeter is = to 1000 micrometers.

150 mega-megadollars??

Mille is Latin for one thousand. It's still heavily used in French and perhaps as much in Italian.

I have literally only ever seen this on HN.

(from the UK where m = million)

you know how a few years back it became popular to say "kk" in online games instead of "k" or "okay"

I suspect people use MM/mm for the same reason, some guy just made a typo one day and it caught on.

kk=okay, cool