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Roman numerals
Individual digits:
I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000
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Add repeated digits, and lesser
digits positioned after greater:
III = 3
VI = 6
VII = 7
VIII = 8
XI = 11
XV = 15
XVI = 16
XX = 20
CXII = 112
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Subtract lesser digits
positioned before greater:
IV = 4
IX = 9
XL = 40
XLIX = 49*
XC = 90
XCIX = 99*
CM = 900
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At most three repeats**, at most
one lesser digit in front of greater:
III = 3
IV = 4**
VIII = 8
IX = 9
XIII = 13
XIV = 14
XVIII = 18
XIX = 19
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1-25:
I = 1
II = 2
III = 3
IV = 4
V = 5
VI = 6
VII = 7
VIII = 8
IX = 9
X = 10
XI = 11
XII = 12
XIII = 13
XIV = 14
XV = 15
XVI = 16
XVII = 17
XVIII = 18
XIX = 19
XX = 20
XXI = 21
XXII = 22
XXIII = 23
XXIV = 24
XXV = 25
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Examples of larger numbers:
MCMLVII = 1957
MCMLXXXVIII = 1988
MCMXCI = 1991
MCMXCIX = 1999*
MM = 2000
MMI = 2001
MMXLVI = 2046
MMCXXIV = 2124
MMDCCXLVII = 2747
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* Lesser digit in front of greater: at most two digits difference, 99 = XCIX (and not IC), 990 = CMXC (and not XM), 1999 = MCMXCIX (and not MIM).
** At most tree repetitions: V, L and D are never repeated. According to older roman principle the digit I can sometimes be repeated more than tree times, for example 4 = IIII on some clocks. M can always be repeated more than three times.
To write very large numbers an extra horisontal line above the digits can be used, multiplying their values by 1000.
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