If a numeral comes after a numeral that is larger or equal in value, then it must be added to the numeral before it.
VI = 5 + 1 = 6
XX = 10 + 10 = 20
A smaller numeral placed before a larger numeral indicates subtraction of that smaller numeral from the larger one.
You cannot subtract more than one value from a Roman numeral.
IV = 5 - 1 = 4
IX = 10 - 1 = 9
3. Max three together
You cannot add more than three of the same Roman numeral together.
III = 3
Wrong: IIII, correct: IV = 4
4. L and D cannot be subtracted
Only powers of ten (I, X, C, M) can be subtracted.
Wrong: VX (10 - 5)
Correct: V (5)
5. One-fifth or one-tenth
You can subtract only value that is 1/5th (one-fifth) or 1/10th (one-tenth) the larger one.
Wrong: IC = 100 - 1 = 99
Correct: XCIX = (100 - 10) + (10 - 1) = 99