[cd34]

  M = 1000 in Roman Numerals
  MM = 1000 1000s, or, 1 million

Years ago, I think it was G. Gordon Liddy that used to refer to billions in a similar manner. i.e. 10 billion dollars was spoken as 10 thousand million so people grasped the enormity of the number.

[matthewowen]

As a Latinist, this makes me cringe. Roman numerals are added, not multiplied. MM = 2000.

Not saying that this isn't the root; just that it is deeply dissatisfying.