The Romans used seven letters to express their numbers. The combination of a letter and its position could represent any number.   They also use a line above the letter, so the numbering system actually represents our own very closely with fourteen different symbols.

The major difference is how they treat the zero.

I This letter is the most familiar and we use it unconsciously to number our chapters in books. It represents a number one.

II If we add two 'I's together it represents the number two.

III And three 'I''s represents the number three.

IIII If you look closely at the old clocks that use Roman numerals, this is exactly what you will see. It makes sense that if you add four 'I''s you'll count till the number four. However this can become cumbersome to represent the number two hundred and one using just two hundred and one 'I''s. So the Romans started using a newer letter for every fifth increment. This is the familiar letter 'V'.

IV You may, however, be more familiar with the 'IV' representation of the number four used in most chapter books. This is a way of representing one less than five. Because the lower value 'I' (one) is written before the higher value 'V' (five), it means take one from five.

V The 'V' on its own represents five.

VI The V followed by a 'I' means five plus one. So this represents six.

VII In the same way V (five) plus II (two) is equal to seven.

VIII And V (five) plus III (three) is equal to eight.

VIIII And V (five) plus IIII (four) is equal to nine. NO! Just when you figured it out the overuse rule catches up with us. Remember the minus rule? For every fifth element we change the code. But just before we change the code, we introduce the code with a smaller letter before it to represent the new code minus one. The next code is 'X' (ten), and we meet it at 'X' (ten) minus 'I' (one), which is the ninth number in our list of numbers.

IX This is the representation for nine.

X And as we already stated, this is the representation for ten.
You should be familiar enough to write down the numbers from ten to twenty using the previous facts.

L represents fifty.

C represents one hundred. In language it is used to represent a century which is one hundred years.

D represents five hundred.

M represents one thousand. And this is the end of our representational list.

V represents five thousand.

X represents ten thousand.

Roman Numeral Practice

Try to do this without looking at the table provided.

 

Make an Adding Machine: Even Adds Roman Numerals

Roman Numerals (Young Math Books)

Sir Cumference & the Great Knight of Angleland: A Math Adventure

The Grapes of Math: Mind Stretching Math Riddles