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The sytem of Roman numerals is a numeral system originating in ancient Rome. It is based on certain letters which are given values:
Throughout the centuries, there has been variation in some of its symbols  specifically in the subtractive notation (which uses, e.g. IV to denote 4 instead of IIII) has entered universal use only in modern times. For example, Forme of Cury, a manuscript from 1390, uses IX for 9, but IIII for 4. Another document in the same manuscript, from 1381, uses IV and IX. A third document in the same manuscript uses IIII, IV, and IX.
The Romans themselves didn't seem to bother that much about what was the correct formation of a number; constructions such as IIX for eight have been discovered. In many cases, there seems to have been a certain reluctance in the use of subtractive notation.
Some rules regarding Roman numerals state that a symbol representing 10^x may not precede any symbol larger than 10^(x + 1); use XCIX not IC for 99.
Use of Roman numerals today is mostly restricted to ordinal numbers, such as volumes or chapters in a book or the numbers identifying monarchs (e.g. Elizabeth II). The BBC uses them to denote the year in which a programme was made. Sometimes they are written using lowercase letters (thus: i, ii, iii, iv, etc.), particularly if numbering paragraphs or sections within chapters. Undergraduate degrees at British universities are generally graded using I, IIi, IIii, III for first, upper second, lower second and third class respectively.
The "modern" Roman numerals, postVictorian era, are shown below:
Roman  Arabic  Notes 

none  0  There was no need of using a Zero 
I  1  
II  2  
III  3  
IV  4  IIII is still used on clock and card faces 
V  5  
VI  6  
VII  7  
VIII  8  
IX  9  
X  10  
XI  11  
XIV  14  
XV  15  
XIX  19  
XX  20  
XXX  30  
XL  40  
L  50  
LX  60  
XC  90  
C  100  This is the origin of using the slang term "Cbill" or "Cnote" for "$100 bill". 
CC  200  
CD  400  
D  500  
CM  900  
M  1000  
ↀ  1000  conjoined C and D, alternative to M 
MCMXLV  1945  
MCMXCIX  1999  Note that there are no short cuts, the I can only precede V or X. 
ↁ  5000  
ↂ  10000  
Ↄ  Reversed 100  Used in combination with C and I to form large numbers 
An accurate way to write large numbers in Roman numerals is to handle first the thousands, then hundreds, then tens, then units.
Example: the number 1988.
One thousand is M, nine hundred is CM, eighty is LXXX, eight is VIII.
Put it together: MCMLXXXVIII.
External link Mathworld on Roman Numerals (http://mathworld.wolfram.com/RomanNumeral)
See also: Numeral system, Arabic numerals, Armenian numerals, Babylonian numerals, Chinese numerals, Greek numerals, Hebrew numerals, Indian numerals, Mayan numerals.
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