Latin numerals

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The Romans didn't use the same numbers as we do (0, 1, 2, 3...), but letters. Each letter had a numeric value:
I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000

Now you can ask how it was possible to write e. g. the number 6 with these letters, since there is no equivalent to it in the list. That was done by combining several letters whose single value had to be added in order to know the combined value. The highest value always has to be placed at the beginning. For example, the Romans would write:
III (1+1+1) = 3
VI (5+1) = 6
MMIII (1000+1000+1+1+1) = 2003

Since this might result in endless collections of letters (99 would be LXXXXVIIII), they made another rule: any letter that has less value than the one placed after it wouldn't be added but substracted from the one after it. So VI (5+1) would be 6 but IV (5-1) would be 4. This rule is usually applied whenever the numeral 4 or 9 appears in our numbers, however you have to be careful when converting from our system to the Roman one: 99 often isn't written as XC (100-10 = 90) and IX (10-1 = 9) but as IC (100-1 = 99), which is easier but unusual for those used to thinking in the decimal system.

Here's an exercise for those who want to practise. The answers can be found at the bottom. Convert the following numbers:
MDI
XL
LX
DXXXVI
CMIX
18
14
56
1600
1998
1999

Answers:
MDI = 1000 + 500 + 1 = 1501
XL = 50-10 = 40
LX = 50+10 = 60
DXXXVI = 500 + 3 * 10 + 5 + 1 = 536
CMIX = 1000 - 100 + 10 -1 = 909
18 = XVIII (10 + 5 + 3 * 1)
14 = XIV (10 + 5 - 1)
54 = LVI
1600 = MDC
1998 = MCMXCVIII
1999 = MCMXCIX or IMM