Why can't you write 49 as IL? I know that it should be XLIV but what's the rule that doesn't let you write it IL? And the same for 490 can't be XD I know it should be CDXC but I want to know the rule.
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You can only put a smaller numeral in front of the two next two numerals to show subtraction, and only if the smaller numeral represents 1, 10, 100, etc.
I is only used before V and X.
X is only used before L and C.
C is only used before D and M.
I is only used before V and X.
X is only used before L and C.
C is only used before D and M.
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1002