Roman    Arabic
  M       1000
  D        500
  C        100
  L         50
  X         10
  V          5
  I          1

The Roman numerals are arranged from left to right in order of decreasing value and added. For example, MMII = 2*1000 + 2*1 = 2002. If a lower numeral appears before a higher one, it is subtracted from the higher value, not added to the total. For example, IX = -1 + 10 = 9. Similarly, XC = -10 + 100 = 90, and XL = -10 + 50 = 40. This avoids writting Roman numerals with more than three consecutive occurrences of the same letter; for example, IV <-- IIII, XL <-- XXXX.

An example:

 
    MCMXLIX =    M        C      M         X    L        I    X
            = 1000 + ( -100 + 1000 ) + ( -10 + 50 ) + ( -1 + 10 ) 
            = 1000 + 900 + 40 + 9 
            = 1949

However, MMMM = 4000 since there is no character for a value larger than 1000.