Ancient Romans used a number system that comprised of the seven letters I, V, X, L, C, D and M. Following table shows the values associated with these letters:
| Roman Numeral | I | V | X | L | C | D | M |
| Arabic Numbers | 1 | 5 | 10 | 50 | 100 | 500 | 1000 |
These seven letters are the base numbers in Roman notation. Other numbers are written with a combination of these seven letters. For example, number 400 is written as CD in Roman notation. There is no zero number in Roman notation.
A number containing two or more digits is created by putting Roman numerals side by side, from highest to lowest. For example:
Rule 1: If a numeral with larger or equal value comes after a numeral -- then it must be added to the numeral before it. For example VI becomes 5+1 = 6. Or III becomes 1 + 1 + 1 = 3.
Rule 2: More than three of the same Roman numeral cannot be added together.
Rule 3: If a numeral with smaller value placed before a numeral -- then it will be subtracted from the from the larger one. For example IV becomes 5 - 1 = 4.
The largest number we can write in Roman numerals is 3,999 which is MMMCMXCIX.
Overline (called Vinculum) notation is often used for writing numbers bigger than 3,999. Vinculum is a bar or overline drawn above the Roman numerals. When used, the underlying Roman numerals are considered to be multiplied by 1,000. For example: