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Roman numeral
A Roman numeral is the name for a number when it is written in the way the Romans used to write numbers. Roman numerals are not used very often today in the west. They are used to write the names of kings and queens, or popes. For example: Queen Elizabeth II. They may be used to write the year a book or movie was made.
The base
I  1
II 2
III3
IV4
V  5
X  10
L  50
C  100
D  500
M  1000
If a lower value symbol is before a higher value one, it is subtracted. Otherwise it is added.
So 'IV' is '4' and 'VI' is '6'.
And for adding 'XIII' makes '13'
For the numbers above X, only the symbol right before it may be subtracted: so 99 is: XCIX (and not IC).
Writing numbers & year
Numbers and years are written as Roman numerals in this way:
Years
 1900 =MCM
 1990 =MCMXC
 1991=MCMXCI
 1992 =MCMXCII
 1993 =MCMXCIII
 1994 =MCMXCIV
 1995 =MCMXCV
 1996 =MCMXCVI
 1997 =MCMXCVII
 1998 =MCMXCVIII
 1999 =MCMXCIX
 2000 =MM
 2001 =MMI
 2002 =MMII
 2003 =MMIII
 2004 =MMIV
 2005 =MMV
 2006 =MMVI
 2007 =MMVII
 2008 =MMVIII
 2009 =MMIX
 2010 =MMX
 2011 =MMXI
 2012 =MMXII
 2013 =MMXIII
 2014 =MMXIV
 2015 =MMXV
 2016 =MMXVI
numbers
 1 =I
 2 = II
 3 = III
 4 = IV
 5 =V
 6 =VI
 7 =VII
 8 =VIII
 9 =IX
 10 =X
 11=XI
 12 =XII
 13 =XIII
 14 =XIV
 15 =XV
 16 =XVI
 17 =XVII
 18 =XVIII
 19 =XIX
 20 =XX
 30 =XXX
 40 =XL
 50 =L
 60 =LX
 70 =LXX
 80 =LXXX
 90 =XC
 100 =C
 200 =CC
 300 =CCC
 400 =CD
 500 =D
 600 =DC
 700 =DCC
 800 =DCCC
 900 =CM
 1000=M
 5000 =V
 10000 =X
 50000 =L
 100000 =C
 500000=D
 1000000=M
Notation
The System that is in use today is: Whenever the same symbol is written four times, it is replaced by subtracting it from the next higher number (5,50,50,500). That way, IV is written instead of IIII (4), XL instead of XXXX (40), etc.
Usually only one number is subtracted, not two. So 8 is always VIII and never IIX
Especially on clocks and watches, IIII can sometimes still be found. This is done partly because the IIII for the 4 o'clock position aesthetically balances the VIII for the 8 o'clock position. Also, some use it as a metaphor that IIII was commonplace in an period of time earlier than the idea of subtraction  IX  was introduced into writing Roman numerals. (4:00 comes earlier in the day than 9:00.)
Proper form is to subtract only a value with the next lower power of 10. Thus, 900 is written CM, but 990 would not be XM  properly, it is CM for the 900 portion and XC for the 90 portion, or CMXC. Similarly, 999 would not be IM but rather CMXCIX  CM for the 900 portion, XC for the 90 portion, and IX for the 9 portion. Only values with 1's are ever used to subtract; 45 is properly XLV, not VL. ^{[1]}
Notations in Roman numerals for numbers higher than 3,001^{[source?]} are rarely seen. One system uses V and X with bars over them to signify 5,000 and 10,000, respectively. So in the lists above, though 5000 etc may appear as V when writing it add a bar on top.
What they are used for
 In the Baltics and Russia, the days of the week, are often written as Roman numbers, I being Monday.
 When writing dates by hand, the month is sometimes written as a Roman numeral, especially for dates written in daymonthyear sequence. Example: 26.XI.2007 = November 26, 2007.
 When movies or books are published, the year of publication or year of copyright may be done as a Roman numeral.
 When people write about Monarchs or Popes, Patriarchs, or other leading figures, they are sometimes counted with Roman numbers, e.g. Queen Elizabeth II (of England), Pope John Paul II, Pope Benedict XVI, Patriarch Alexius II (of the RussianOrthodox church)
 In France, the trimesters are sometimes counted with Roman numerals.
 In Poland, roman numerals are used to show the month in dates and as a short method of writing ordinals (i.e. VI to be 6th)

