From a recent pub quiz - - In Roman numerals, why is IC not the correct answer for 99?

The Romans usually wrote IIII for 4 and XXXX for 40. The number 949 was DCCCCXXXXVIIII. To shorten the length of such numbers a "subtraction rule" appeared in later Roman times and was commonly used in medieval times. The "subtraction rule" allows the use of six compound symbols in which a smaller unit precedes the larger:

IV = 4 IX = 9 XL = 40 XC = 90 CD = 400 CM = 900

Using these symbols, 949 is written more compactly as CMXLIX. (Other "subtracted" symbols are not allowed. Thus 99 must written XCIX, not IC.) The use of subtracted symbols was never mandatory, so IIII and IV can be used interchangeably for 4.

i. Only powers of ten (I, X, C, M) can be subtracted.
ii. The smaller letter must be 1/5th or 1/10th of the larger one.
iii. The smaller letter must either be the first letter or preceded by a letter at least ten times larger than it.
iv. If another letter follows the larger one, it must be smaller than the number preceding the larger one.