Roman Numerals Explained
Stephen P. Morse, San Francisco
Roman Numerals are a way of encoding numbers using seven letters in the Latin alphabet. The seven
letters and their associated numeric values are as follows:
I = 1
There are very specific rules for how the letters are selected.
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000
1. Starting with the highest valued letter (M), keep repeating that letter as long as the sum of
all the letters does not exceed the numeric value desired.
2.Repeat step 1 for each lower-valued letter in turn.
3. Four repetitions of the same letter are not allowed. So make the following replacements
(these are referred to as subtractive notation)
IIII not preceded by V => IV
4. Exception: The number 4 on clocks is sometimes written as IIII instead of IV.
VIIII => IX
XXXX not preceded by L => XL
LXXXX => XC
CCCC not preceded by D => CD
DCCCC => CM
MMMM is not permitted (the highest possible number is 3999)
III = 3
IV = 4
IX = 9
XXV = 25
XCIX = 99
MCMLXXXIV = 1984
1. Large numbers are sometimes written with a bar over them (Vinculum notation).
For example, V̅=5000, X̅=10000, D̅=50000, C̅=100000, etc.
2. Large numbers are sometimes written with archaic symbols.
For example, ↁ=5000, ↂ=10000, ↇ=50000, ↈ=100000, etc.
3. A modern version of the archaic symbols is the Apostrophus notation.
© Stephen P. Morse, 2018