DAK why 45 is written XLV and not VL?

"Roman Numerals

"Subtractive Principle:

"Any numeral is counted positively unless there's a larger numeral anywhere to its right, in which case it is counted negatively. However, proper Roman numbers are subject to the following restrictions about the applicability of the subtractive principle.

"The use of the subtractive principle has always been optional. Its systematic use is fairly modern. For example, it's acceptable to use IIII instead of IV, as is usually done on clockfaces (to "balance" their left and right halves, so we're told).

"The subtractive principle (a subtrahend preceding a minuend) may apply:

• Only to a numeral (the subtrahend) which is a power of ten (I, X or C).
For example, "VL" is not a valid representation of 45 (XLV is correct)."

- http://home.att.net/~numericana/answer/roman.htm

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