Roman numerals were used in Europe until the 9th century when Arabic numerals became more popular. The system was inspired by Attic numerals used in Greece and adapted by the Etruscans. The Romans changed the symbols and added some, requiring the reader to add them together to discern the number. Subtractive notation is used for numbers like four, where a lesser value symbol is placed in front of a greater value symbol.
Roman numerals are a common numbering system throughout Europe until about AD 900, when Arabic numerals supplanted Roman numerals for most applications. Roman numerals can still be seen on formal documents to mark the date or used to iterate simple numbers, such as those used on an outline. For mathematical purposes, however, these numbers have long since been discarded, because they are clunky and difficult to work with compared to Arabic numerals.
The inspiration for Roman numerals is found in Attic numerals, which were used in Greece around the 7th century BC. Attic numerals used symbols to represent the numbers one, five, 10, 100, 1,000 and 10,000. The representations of the symbols likely came from counting sticks which would have been used to mark goods as they were counted. The Etruscans, found in what is now Italy, picked up the Attic number 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 Etruscan numbers as well as adding some. Under 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 1,000 . 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 the symbols together to discern the number being represented. This can be time consuming, especially with numbers as large as MMMDCXIII, which reads as 1.000 + 1.000 + 1.000 + 500 + 100 + 10 + 1 + 1 + 1 or 3.613.
However, numbers such as four are not, by convention, written in Roman numerals such as IIII. Instead, a system called subtractive notation is used. Subtractive notation means that if a lesser value symbol is placed in front of a greater value symbol, the lesser symbol must be subtracted from the greater one. Thus, IV means four, just like MCMLXXXIV means 1984. When using subtractive notation, only multiples of 10 are used, so VC times 95 would be incorrect and XCV would be correct. 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.
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