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Roman Numerals
Writing Numbers using Letters
The Romans used letters to write their numbers, each letter standing for a different value. The values of all the letters were then added together to make the number. The letters (usually written in CAPITALS but they can also be lower case) they used were:
M m 
D d 
C c 
L l 
X x 
V v 
I i 
1000  500  100  50  10  5  1 
The order of the letters was usually from the largest down to the smallest (left to right) and now that is a fixed rule for writing Roman numbers. For instance:
13  would be written as  XIII 
2003  would be  MMIII 
99  would be  LXXXXVIIII 
1998  is  MDCCCCLXXXXVIII 
2000  is  MM 
The Subtraction Rules
Instead of just adding the values, subtraction is nowadays used too (and was occasionally used by the Romans too). The rule is:
if a smaller tens value (I, X or C) appears before one of the next two larger ones: I before V or X X before L or C C before D or M it was to be subtracted not added. 
So I could appear before V or X to make IV for 4 and IX represents 9 but VI is 6 and XI is 11 by the normal (addition) rule. I could not appear before any of the other letters so it is incorrect to write IL to mean one less than 50 for 49 and IC is not correct for 99 nor is IM for 1999.
Similarly, X could appear before L or C to give XL for 40 and XC for 90 (but not before any other letter). Also C could be written before D or M for 400(CD) and 900 (CM).
Can you... Write out all the Roman Numbers from 1 to 50

 Convert these numbers to Roman numerals:
 49 (hint: it is 40 + 9)
 99 (hint: it is 90 + 9)
 199
 299
 399
 499
Why don't we use Roman Numerals today?
We use a system of 10 digits: 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 to write our numbers, not the Roman system. Why?
Try writing the following in Roman numbers:
 the speed of light: 300,000,000 metres per second (or 300,000 km per sec)
 the mass of the Sun: 1·989 10^{30} kg
 the diameter of the Sun: 1,390,000 km
 the distance of the Earth from the Sun: 149,600,000 km
Also, try multiplying two numbers using only Roman numerals!
Can you...find examples of Roman Numerals in each of the following places: on the foundation stone of an old building or bridge
 on a plaque commemorating the opening of a building or bridge
 on the final part of the credits after some TV programmes, showing when the film or programme was made
 as page numbers of the preface in a book, which is the initial pages before the actual text of the book starts.
 as a way of numbering lists (like this one!)
 on some clocks (e.g. Big Ben in London).