The third rule - Subtraction of basic symbols in subtractive notation of a Roman numeral (placing lower value symbols in front of larger value ones)
Numerals subtraction rule in subtractive notation:
A numeral of a lesser value placed in front (next to the left) of a larger value numeral is subtracted from the larger one. This is called subtractive notation.
Only these symbols are allowed to be used in subtractive notation: I (1), X (10), C (100), M (1,000), (X) (10,000), (C) (100,000), (M) (1,000,000).
Numerals V (5), L (50), D (500), (V) (5,000), (L) (50,000), (D) (500,000) are NOT allowed to be used in subtractive notation.
Important: Please also see the next rule, no. 4.
Number four (4) is written using two important symbols listed under the first rule, I (1) and V (5): subtracting I from V, by placing I next to the left of the V symbol. We thus obtain: 4 = 5 - 1 = V - I = IV.
Number nine (9) is written using two important symbols listed under the first rule, I (1) and X (10): subtracting I from X by placing I next to the left of X symbol. We thus obtain: 9 = 10 - 1 = X - I = IX.