Roman Numeral rules

What rule will explain that 49 is XLIX and not IL?

I guess that symbols can only be subtracted by symbols of one level lower.

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Originally Posted by Roman numerals wiki

A numeral for 10n (I, X, or C) may not precede a numeral larger than 10n+1, where n is an integer.[citation needed] That is, I may precede V and X, but not L or C; X may precede L or C, but not D or M. The numerals 5×10n (V, L, or D) may not be followed by a numeral of greater or equal value.[citation needed] Any symbol that appears more than once consecutively may not be followed by a symbol of larger value.

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Ask Dr.Math...

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Here are the official rules for subtracting letters:

* Subtract only powers of ten, such as I, X, or C. Writing VL for 45 is not allowed: write XLV instead.
* Subtract only a single letter from a single numeral. Write VIII for 8, not IIX; 19 is XIX, not IXX.
* Don't subtract a letter from another letter more than ten times greater. This means that you can only subtract I from V or X, and X from L or C, so MIM is illegal.

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Slightly off-topic but does anyone know why 4 = IV except on Clock Faces when it's IIII ?

'cause it looks more symetrical (4 times values using only I, 4 times values using V and four times values using X).

It's not on all clock faces, actually. Apparently a long time ago it was only on Louis XIV clocks because he preferred IIII over IV (and he also preferred to be called Louis XIIII). But I know that every clock I've seen uses IV...