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Roman numerals are a method of writing numbers employed primarily by the ancient Romans. It place of digits, the Romans used letters to represent the numbers central to the system:
| $I$ |
$1$ |
| $V$ |
$5$ |
| $X$ |
$10$ |
| $L$ |
$50$ |
| $C$ |
$100$ |
| $D$ |
$500$ |
| $M$ |
$1000$ |
Larger numbers can be made by writing a bar over the letter, which means one thousand times as much. For instance $\overline{V}$ is $5000$ .
Other numbers were written by putting letters together. For instance $II$ means $2$ . Larger letters go on the left, so $LII$ is $52$ , but $IIL$ is not a valid Roman numeral.
One additional rule allows a letter to the left of a larger letter to signify subtracting the smaller from the larger. For instance $IV$ is $4$ . This can only be done once; $3$ is written $III$ , not $IIV$ . Also, it is generally required that the smaller letter be the one immediately smaller than the larger, so $1999$ is usually written $MCMXCIX$ , not $MIM$ .
It is worth noting that today it is usually considered incorrect to repeat a letter four times, so $IV$ is preferred to $IIII$ . However many older monuments do not use the subtraction rule at all, so $44$ was written $XXXXIIII$ instead of the now preferable $XLIV$ .
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