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The system of Roman numerals is a numeral system originating in ancient Rome. It is based on certain letters which are given values:

When describing members of a list, first A, B, C, D tend to be used, then 1, 2, 3 then i, ii, iii, iv

Throughout the centuries, there has been variation in some of its symbols — specifically, the subtractive notation (which uses, e.g., IV instead of IIII to denote 4) 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 10x may not precede any symbol larger than 10x + 1; use XCIX not IC for 99.

Clock faces typically show IIII for 4 o'clock and IX for 9 o'clock — using the subtractive principle in one case and not in the other. It has been said that the reason 4 o'clock is IIII (and not IV), is because IV is the first two letters of IVPITER, the supreme god of the Romans.

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 lower-case 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, post-Victorian era, are shown below:

RomanArabicNotes
none0There was no need for a zero.
I1
II2
III3
IV4IIII is still used on clock and card faces.
V5
VI6
VII7
VIII8
IX9
X10
XI11
XII12
XIII13
XIV14
XV15
XIX19
XX20
XXX30
XL40
L50
LX60
LXX70
LXXX80
XC90
C100This is the origin of using the slang term "C-bill" or "C-note" for "\$100 bill".
CC200
CD400
D500
CM900
M1000
1000Conjoined C and D, alternative to M.
MCMXLV1945
MCMXCIX1999Note that there are no short cuts, the I can only precede V or X.
MM2000
MMM3000
5000
10000
Reversed 100Reversed C, 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.

Unicode has a number of characters specifically designated as Roman numerals, most of them only included for compatibility with East Asian standards. They range from U+2160 to U+2183. For example, MCMLXXXVIII could alternatively be written as ⅯⅭⅯⅬⅩⅩⅩⅧ or ⅯⅭⅯⅬⅩⅩⅩⅤⅠⅠⅠ (this requires a font, such as Arial Unicode MS, that contains the necessary characters).