What Are Roman Numerals?
Roman numerals, developed by the ancient Romans, are symbols that stand for numbers. They are written using seven basic symbols: 1=1, V=5, X=10, L=50, C=100, D=500, and M=l,000. Historic evidence indicates that C is from the Latin word "centum," meaning "one hundred"; and M is from the Latin word "mille," meaning "one thousand." Sometimes a bar is placed over a numeral to indicate that it is 1,000 times greater. For example, 5,000 may be written as V.
Roman numerals are an additive system. This means that the numerals are written in a descending order from left to right and added together, to determine each digit.
Take the following example: MMCCCXXXII
2000 300 30 2
By adding these numbers, we come up with 2,332.
Roman numerals, however, may also be a subtractive system. This is the case when a smaller numeral is placed in front of a larger one. Then the smaller numeral is subtracted from the larger one. Examples of this notation include: IV=4; IX=9; XL=40; and XC=90.
Thus, using Roman numerals it's possible to express a number in more than one way. For instance, the number 1997 can be written (in actuality there are no spaces between characters in Roman numerals):
M DCCCC LXXXX VII or M CM XC VII
1 900 90 7 1000 900 90 7
Source: World Book Encyclopedia, vol. 16, pp. 413-14.