Guide to Roman numerals

Just thought I might write a guide here, as I think many people today aren't always correct when speaking about ancient Roman Numerals, as used by the Romans. For example, the use of the character M is post classical, and was almost never used by the Romans.

The Romans used symbols to represent numbers:

I = 1 (the basic single down slash)
V = 5
X = 10
L = 50
C = 100

As most people know, numbers are added when one is placed behind another of equal or greater value, and if its placed behind another of less value, it is subtracted by that lesser value. For example, IIII is 4, VI is five. IX is 9. You may have noticed that IV also makes 4, just like how IIX and VIII both make 8. In numbers that "borderline"the subtraction and addition method, both forms would have been considered acceptable. Roman numerals as a system was not very standardized, especially when you go to higher numbers:

D or = 500
Either a strikethrough D or I and a flipped C, called an apostrophus
or ∞ or ( | ) = 1000
notice that adding another I to doubles it to 1000. Sometimes, it's written less letter-like and more like a circle with a line through it: ( | ). the infinity symbol is also used as 1000.  Adding a bar above the letters multiplies it by 1000, so for example,
_
I   = 1000
_
V or I or | ) = 5000
CCI or ((| )) or X(with bar)= 10000   eg. ((| )) ((| )) ((| )) = 30000


And this pattern continues... sometimes the ( or ) are replaced by shorter vertical lines, such as  ||||| for 10000. Usually, when large numbers are inscribed, they are written smaller than normal character. Another variation is to have a circle with a verticle line through the middle, and have the ( and ) written inside the two halves of the circle. A million is represented by X enclosed with lines:
__
|X |  = 1,000,000 The three lines represents mulitplcation by 100,000.

There are also some instances where the lateral lines were only half way drawn, but it isn't very clear if it means something different.

There are many exceptions to the rules, such as dealing with military numbers.



Edited by Imperator Invictus

M can also equal 1,000.  I did not know a lot of that, actually.  Thank you.

Apparantly, it's

Known as the semis. For other fractions, the Romans didn't have a widely used symbol for them, as far as I know. They had words to represent numbers like 5/12 and composite phrases that would say "three fifths" but not a symbol.

Mathmaticians during the Roman period usually used Greek, eygptian, and other systems more flexible than the Roman one. Greek numerals were capable of fraction forms.