Roman Numerals
In Europe it's quite common to see Roman numerals written on monuments or churches.
But you can also see them on the face of a watch or a clock.
Symbols
Ancient Romans used the following seven symbols to define numbers:
Rules
Read Roman numerals from left to right.
Any number containing more than one digit is written appending the Roman numeral equivalent for each, from highest to lowest.
In a sequence of Roman numerals, if a numeral is greater or equal than the following, add their values.
Examples: III = 1 + 1 + 1 = 3 MMC = 1000 + 1000 + 100 = 2100 XVII = 10 + 5 + 1 + 1 = 17
If a numeral is smaller than the following one, subtract the value of the left one from the value of the right one. The subtraction only applies to these cases:
∙ I can be subtracted only from V and X. Examples: 4 = IV, 9 = IX
∙ X can be subtracted only from L and C. Examples: 40 = XL, 90 = XC
∙ C can be subtracted only from D and M. Examples: 400 = CD, 900 = CM
More: IX = 10 - 1 = 9, XL = 50 - 10 = 40, XXIV = 10 + 10 + (5 - 1) = 24
Numbers that don't include these special cases must be written using addition.
Only numerals I, X, C and M can be repeated next to each other, usually no more than three times.
Examples: MMDX = 1000 + 1000 + 500 + 10 =2510, CCCV = 100 +100 + 100 + 5 = 305
Numerals V, L and D can be used only once, and are never subtracted.
Large Numbers, Zero and Historical Notes
A Roman numeral with a bar over it means that the number is multiplied by 1000.
Example:
The example above shows that "zero" didn't have a special symbol to represent it, because the Roman system was not positional as the Arabic one (the most used nowadays) is.
Historical note: subtraction was introduced during the Middle Ages. Before that, Roman numerals could only be added. This is why over some entrances of the Colosseo in Rome you can see the number 4 written as IIII instead of IV.