Roman numerals are a numbering system that uses letters to represent numbers.

Roman numerals are a numbering system that was widespread across Europe until approximately 900 AD, when Arabic numerals supplanted Roman numerals for most applications. Roman numerals can still be seen in formal documents to mark the date or being used to iterate over simple numerals such as those used in an outline. For mathematical purposes, however, these numerals have long since been discarded, because they are clunky and difficult to work with compared to Arabic numerals.

Roman numerals were used throughout Europe until around 900 CE.

Inspiration for Roman numerals can be found in Attic numerals, used in Greece around the 7th century BCE. Attic numbers used symbols to represent the numbers one, five, 10, 100, 1,000, and 10,000. The depictions of the symbols likely came from sticks that would have been used to mark goods as they were counted. The Etruscans, located in what is now Italy, took the Attic numeral system and adapted it for their own use, creating symbols to represent one, five, 10, 50, and 100.

The Romans changed the symbols used for the Etruscan numerals, as well as adding some. In the Roman numeral system, I stands for one, V stands for five, X stands for 10, L is used for 50, C stands for 100, D stands for 500 and M is 1000. A slash placed over a symbol multiplies its value by 1000 . All numbers in Roman times would have been written using these symbols. 17, for example, would have been written as XVII. As can be seen, this system requires the reader to add up the symbols to discern the number being represented. This can be very time consuming, especially with large numbers like MMMDCXIII which shows 1000 + 1000 + 1000 + 500 + 100 + 10 + 1 + 1 + 1 or 3613.

However, numbers like four are not, by convention, written in Roman numerals like IIII. Instead, a system called subtractive notation is used. Subtractive notation means that if a lower-valued symbol is placed in front of a higher-valued symbol, the smaller symbol must be subtracted from the larger one. Hence, IV means four, just as MCMLXXXIV means 1984. When using subtractive notation, only multiples of 10 are used, so VC for 95 would be incorrect and XCV would be adequate. It is also considered improper to skip orders of magnitude when using subtractive notation, meaning that XM would not be used to represent 990, but CM could be used to denote 900.