The Romans used a different system of numbers to the one we use today; instead of the digits 0 to 9 they used symbols to represent different values:
The symbols are normally arranged in descending order and the values are simply added together.
Where a symbol of lesser value comes before a symbol of greater value then the lesser value is subtracted.
For example IV actually means 5 - 1 = 4 and CMII means 1000 - 100 + 1 + 1 = 902
Only C, X and I can be subtracted and only one smaller symbol can be used.
The symbol used for subtraction must not be less than a tenth of the value of the number it precedes.
So something like IIX or LM is NOT allowed. (IIX uses two smaller numbers (II) before the larger one (X) and L(50) is less than one tenth the size of M(1000))
This means that: