Roman Numerals: The Rules

There are three basic rules to remember:

1. No symbol is used more than 3 times in a row:

No symbol is used more than 3 times in a row:

2. If a symbol appears after a larger or equal symbol, it is added to it:

Note: This works in all cases except $VV$ . The symbol $X$ is used to represent the number 10 therefore $VV$ is not needed.

3. When the left-hand symbols are smaller than the right, subtract them from the right-hand symbol:

Note:
We can only subtract one character from another, so $IIX$ would not be one and one from ten = 8.
Instead, to get $8$ we would have to use $VIII$ meaning $5+1+1+1=8$ .

We can start counting and apply the rules we learnt using the sentence:

'I Value Xylophones Like Cows Do Milk'.
$I=1$ , $V=5$ , $X=10$ , $L=50$ , $C=100$ , $D=500$ , $M=1000$

$I$
$II$
$III$
$cancel(IIII)$ – No symbol is used more than 3 times.
$IV$ (1 from 5 = 4) – when left is small and right is bigger, subtract left from right-hand figure.
$V$
$VI$ (5 add 1 = 6) – If a symbol appears after a larger or equal figure it is added.
$VII$ (5 add 1 add 1 = 7) – If a symbol appears after a larger or equal figure it is added.
$VIII$ (5 add 1 add 1 add 1 = 8) – If a symbol appears after a larger or equal figure it is added.
$cancel(VIIII)$ – No symbol is used more than 3 times.
$IX$ (1 from 10 = 9) – when left is small and right is bigger, subtract left from right-hand figure.
$X$

This pattern continues for every number.