Roman numerals

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:

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 $XLIV$.