Latin course for the Virtual School of Languages

Roman numerals

The Romans didn't use the same numbers as we do (0, 1, 2, 3...). Our numbers
were derived from Arabic during the Middle Ages.
The Romans used letters instead and gave each letter 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 = 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.

As an exercise, convert the following numbers (scroll down for the answers):
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