1999 in Roman Numerals

What was the year “1999” in Roman Numerals? Why wasn’t it “MIM” (a thousand, plus one less than a thousand)?

I always thought the rule was that you could only “subtract” units that were one order of magnitude less —

IX = 9, XC = 90, CM = 900, but not IC = 99 or IM = 999.

If I’m correct then 1999 would be MCMXCIX – which still has a certain elegance, IMO, if not the symmetry of MIM.

And would the Y2K problem have been the AMM problem in Rome?

Here’s the answer from Cecil:
http://www.straightdope.com/classics/a3_375.html

Interestingly (to me anyway), this is the second time in one hour that I’ve posted this link to the GQ forum.

Arjuna34