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 is .

Other numbers were written by putting letters together. For instance means . Larger letters go on the left, so is , but 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 is . This can only be done once; is written , not . Also, it is generally required that the smaller letter be the one immediately smaller than the larger, so is usually written , not .

It is worth noting that today it is usually considered incorrect to repeat a letter four times, so is preferred to . However many older monuments do not use the subtraction rule at all, so was written instead of the now preferable .