Roman Numbers 
The numbers that we use are called Arabic numbers, as the Europeans learnt them from the Arabs, although they were invented in India. Romans had a completely different number system.
Click in the first box and type in a whole positive number less than 4000. The Roman number will appear in the other box. Try different numbers, and see how the Romans would write them. If you have a Roman number and you want to find out what it is, click on the Roman box and type it in.

In Arabic numbers, we only have ten symbols (0123456789)
which can give any number however large, although the bigger numbers get
quite long. So 1 means one, and 100 means a hundred. The symbol "1" means
something different depending if it has any numbers after it.
The Romans thought in a different way. One is I, and two is II, and three is III. Five has a different symbol, V. There were different symbols for ten, fifty, hundred, five hundred and thousand. However, there were no symbols for anything higher, so they could only describe numbers up to 3999. Above this, there were various ways to describe numbers, but no generally agreed way. 
MDCCCLXXXVIII is 1000+500+100+100+100+50+10+10+10+5+1+1+1 or 1888
MCMXCIX is M CM XC IX or 1000+(1000100)+(10010)+(101) or 1999
You have to be good at adding and subtracting for Roman numbers! Also, you can't tell from the length of the number, how big it is (MM is 2000).
But why did the Romans choose these letters as symbols?
Well, the Romans counted on their fingers! I or II or III are different
numbers of fingers held up. So what are five fingers? A whole hand, of course!
If you look at a right (see right), you can see that the thumb and little finger
make a V, and it's a lot easier than to draw the whole hand.
Ten
fingers are both hands, and X is two V's (with one upside down).
The
Romans spoke a language called Latin, and the Latin for hundred is Centum. So C
stands for hundred.
What do these words mean? Century  Centurion 
Centimetre  Cent  Centipede
The Latin for thousand is Mille. So M
stands for thousand.
What do these words mean? Millennium  Millimetre 
Millipede
Now for fifty. Fifty is half of a hundred, so the Romans
took the symbol for hundred, C, and cut it in half. This makes an L, which
became fifty.
Five hundred is half of a thousand, so the Romans took the symbol
for thousand, M, and cut it in half the other way. This makes a (sort of) D,
which became five hundred.
One place where you
often see Roman numbers is on a clock face. The hours are marked as I to XII.
However, there is something odd about these Roman numbers. If you look at four,
it is IIII instead of IV. I think that this is because half of the numbers are
upside down, since they follow the edge of the clock face round. You can get IV
and VI muddled up when they're the right way up. It is even worse when they're
upside down! IX and XI are not such a problem, since they are more or less the
right way up. In fact, the Romans never had clocks like this, since they had not
been invented.
This is the accepted modern way to count with Roman numbers. The Romans
themselves were not so fussy. There is a Roman tombstone in York, England, of
Lucius Duccius Rufinius, who was the standard bearer of the VIIII legion (9th),
and was XXIIX years old (or 28!)
At the top of this page, when using the
convertor, I tell you to type a number in which is less than 4000. Why? When we
use Roman numbers today, we don't use them for big numbers, so you never see the
Roman number for 5000 (and if you don't have that, then you can't write 4000).
The Romans agreed on symbols for 1, 5, 10, 50, 100, 500, and 1000, but there
were different symbols for 5000 and also for the bigger numbers. Here is one way
the Romans wrote big numbers.
As you can
see, it's getting quite messy!