In Roman numerals, M equals 1,000, D equals 500, C equals 100, L equals 50, X equals 10, and I equals one.
When a larger number is followed by a smaller number, such as XI, the numbers are added. Thus, XI = 10 + 1 = 11.
When it is the opposite, such as IX, the smaller number is subtracted from the bigger number. So, IX = 10 - 1 = 9.
When using multiple numbers of the same value, no more than three of the same symbol should appear together. If four are needed, it should be that value subtracted from the next-largest value. Examples include XL = 40, IV = 4 and CD = 400.
For each value above one, placing a bar or vinculum above the number multiplies it by 1,000. For instance, an X with a vinculum is 10,000.