Roman numerals to 1000
In a nutshell
Before the numbers that are used today existed, the Romans used letters to express positive numbers.
100, 500 and 1000
Similarly to writing numbers up to $100$, the numbers beyond $100$ are 'built' using a combination of capital letters. The letters used to represent $100, 500$ and $1000$ are as follows:
$100$
 $500$
 $1000$

C
 D
 M 
Every other number between 100 and 1000
To 'build' a number using Roman numerals, work out the symbols for the ones, tens and hundreds, writing each combination of letters in size order from left to right.
The rules associated with larger Roman numerals are the same as those associated with smaller ones.
Rule 1.
Numbers represented by symbols are added up from left to right.
Example 1
DCII is equal to $500+100+1+1=602$
Rule 2.
The symbols are written and added in order, left to right, from largest possible values.
Example 2
$812$ is represented by DCCCXII $=500+100+100+100+10+1+1$ and not CCCCCCCCVVII $=100+100+100+100+100+100+100+100+5+5+1+1$.
Rule 3.
Where a smaller number is placed to the left of a larger one, take away from the value on the right. Symbols cannot be used more than three times in a row.
Example 3
$400$ is represented by CD $=500100=400$ and not CCCC $=100+100+100+100=400$.
Tip: Remember that $1,5,10$ and $50$ in Roman numerals are I, V, X and L.
Writing years in Roman numerals
Just like any other numbers, years can be written in Roman numerals by using the above rules.
Example 4
What is $1999$ in Roman numerals?
'Build' the number using the following:
$\begin {array}{cccccccc}1999&=&\text{Thousands}&+&\text{Hundreds}&+&\text{Tens}&+&\text{Ones}\\1999&=&1000 &+&900&+&90&+&9 \\\end{array}$
Work out the correct symbols for each number using the above rules.
$1000=\text{M}\\900=\text{CM}\\90=\text{XC}\\9=\text{IX}$
Write the letters in order from left to right.
$\text{MCMXCIX}$
Numbers greater than 3000
The final rule used in Roman numerals is as follows:
Rule 4.
For multiples of $1000$ greater than $3000$, use a line above the symbols for the corresponding ones values.
Example 5
The multiples of $1000$ up to $10\space000$ are listed below:
$\bold{1000}$
 $\bold{2000}$
 $\bold{3000}$
 $\bold{4000}$
 $\bold{5000}$
 $\bold{6000}$
 $\bold{7000}$
 $\bold{8000}$
 $\bold{9000}$
 $\bold{10\space000}$

$\text{M}$
 $\text{MM}$
 $\text{MMM}$
 $\overline{\text{IV}}$
 $\overline{\text{V}}$
 $\overline{\text{VI}}$
 $\overline{\text{VII}}$
 $\overline{\text{VIII}}$
 $\overline{\text{IX}}$
 $\overline{\text{X}}$
