Try our Roman Numeral Challenge.
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).
More on Roman Numerals from Infoplease: