Roman numerals

[blackfive]

I recently watched a TV film made in 1999 and at the end of the credits the date was shown as MCMXC1X. It's something I pondered about at the time but never bothered asking. Is there any reason why it couldn't be simply written as M1M?

[Quizmonster]

Click http://www.jimloy.com/arith/roman0.htm for a website that deals - not very decisively - with exactly this point.

[Ewood27]

I have no idea how the convention came about, but if you look at normally-written Roman numerals you will find that where a small number is given before a larger one to indicate 'less than', this is only applied to numbers one or two steps larger, not more. Reading that, it's about as clear as mud, so some examples. I can be used before V or X (IV and IX) but not before L, the next number up. 49 is XLIX, not IL. Similarly you can have XL and XC, but not XD or XM. MIM just isn't a valid number under that convention.