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Roman numerals are expressed by letters of the alphabet and are rarely used today except for formality or variety. There are four basic principles for reading Roman numerals:
- A letter repeated once or twice repeats its value that many times (XXX = 30, CC = 200, etc.).
- One or more letters placed after another letter of greater value increases the greater value by the amount of the smaller (VI = 6, LXX = 70, MCC = 1200, etc.).
- A letter placed before another letter of greater value decreases the greater value by the amount of the smaller (IV = 4, XC = 90, CM = 900, etc.). Several rules apply for subtraction: (a) only subtract powers of ten (I, X, or C, but not V or L); (b) only subtract one number from another; (c) do not subtract a number from one that is more than 10 times greater (that is, you can subtract 1 from 10 [IX] but not from 20—there is no such number as IXX).
- A bar placed on top of a letter or string of letters increases the numeral's value by 1,000 times (XV = 15, = 15,000).
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