IOTN

This is the Top Left hand side of the IOTN ruler.  It is used for measuring positive overjets.

The "i" is used with incompetent lips - the grade is then 3a.  The "c" is used for competent lips - the grade is then 2a.

Click on the image to see how to use it.

The number 2 at the far left hand side belongs to the bottom left portion of the ruler.