As its name indicate, Roman numbers or numerals were developed in Europe during and after the Roman empire. Romans needed a notation system to calculate, so they chose letters to write their figures.
One curious thing you must learn, however, is that, despite the fact today Roman numerals are...
I – V – X – L – C – D – M
...they were not always like that.
Why those letters and not others?
I found something interesting about the origin of Roman numerals. According to my reading, the symbols were chosen because...
+ I resembled a finger pointing up
+ V resembled an open hand, or five fingers
+ X our two arms crossed in front of ourselves, representing our ten fingers
+ L resembled our hand in an L position; that is, with the thumb extended and our other four fingers together; while,
+ C was a contraction of the Latin word centum, hundred.
I did not find why they chose the letter D for five hundred, but I did find they did not use the letter M for the thousand. This usage appeared somewhat later, during the first part of the Middle Ages.
How did they represent larger numbers then? They used either parenthesis or a bar on top of the number. These symbols indicated you needed to multiply the figure by 1000. For example, two thousand was represented like this: (I)(I). It is not indicated like that any more.
Besides, they represented a number four with four one symbols (IIII). The subtracted way we know today (IV) appeared later.
Roman numbers today
Nowadays, Roman numbers are not used to carry out mathematical operations, so this shouldn't become a big deal for anybody. We write Roman numbers from the greater amount to the smaller one, and using a maximum of three equal symbols in a row. Instead of using a fourth equal symbol, we indicate numbers four or nine placing the next number and a unit of the previous kind in its left, indicating a subtraction.
Romans did not use the figure zero, so there isn't a Roman numeral for it.
Thus, we have:
+ one = I
+ two = II
+ three = III
+ four = IV (five minus one)
+ five = V
+ six = VI (five plus one)
+ seven = VII
+ eight = VIII
+ nine = IX (ten minus one)
+ ten = X
+ thirty = XXX
+ forty = XL (fifty minus ten)
+ a hundred forty-seven = CXLVII
+ a thousand nine hundred seventy-nine = MCMLXXIX
...And, for numbers over three thousand, we respect the bar Romans used in the past, indicating that the number must be multiplied by one thousand:
+ two thousand = MM
+ three thousand = MMM
+ four thousand five hundred sixty-six = IVDLXVI
Interesting, isn't it?
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