The Romans were active in trade and commerce, and from the time of learning to write they needed a way to indicate numbers. The system they developed lasted many centuries, and still sees some specialized use today.
Today, Roman numerals traditionally indicate the order of rulers or ships sharing the same name (e.g. Queen Elizabeth II). They are also sometimes still used in the publishing industry for copyright dates, and on cornerstones and gravestones when the owner of a building or the family of the deceased wishes to create an impression of classical dignity.
For instance, this page was written in the year of Nova Roma's founding, 1998 CE. That year is written as MCMXCVIII. Nova Roma, following ancient Roman traditions, counts years from the founding of Rome, ab urbe condita. By that reckoning Nova Roma was founded in 2751 a.u.c. or MMDCCLI a.u.c..
The big differences between Roman and Arabic numerals (the ones we use today) are:
- Romans didn't have a symbol for zero
- numeral placement within a number can sometimes indicate subtraction rather than addition.
Here are the basics
- The easiest way to note down a number is to make that many marks - little I's. Thus I means 1, II means 2, III means 3.
- The symbol for 5 is V. Placing I in front of the V — or placing any smaller number in front of any larger number — indicates subtraction. So IV means 4. After V comes a series of additions - VI means 6, VII means 7, VIII means 8.
- X means 10. IX means to subtract I from X, leaving 9. Numbers in the teens, twenties and thirties follow the same form as the first set, only with X's indicating the number of tens. So XXXI is 31, and XXIV is 24.
- L means 50. 40 is XL since it indicates 10 subtracted from 50. And thus 60, 70, and 80 are LX, LXX and LXXX.
- C stands for centum, the Latin word for 100. We still use this in words like "century" and "cent." The subtraction rule means 90 is written as XC. Like the X's and L's, the C's are added to the beginning of numbers to indicate how many hundreds there are: CCCLXIX is 369.
- D stands for 500. CD means 400. So CDXLVIII is 448.
- M is 1,000, and stands for Latin mille. You see a lot of Ms because Roman numerals are used a lot to indicate dates.
Larger numbers were indicated by putting a horizontal line over them, which meant to multiply the number by 1,000. Hence a V with a line over the top means 5,000. This usage is no longer current, because the largest numbers usually expressed in the Roman system are dates, as discussed above.