What is the proper use of "m" designations to abbreviate million?

Question

I learned that a single M (such as $1M) is to be read as one thousand since the M stands for "mil" which is latin for Thousand. Therefore one would use two m's $1mm to indicate one million (double mil).

Yet I see people using M to indicate one million?

Can explain the right usage?

Answer 1

Read this, but MM is my experience has always been for million and M for thousand.

http://en.wikipedia.org/wiki/M