Protractor Postulate
If one ray of an angle lies on 0, then the measure of the angle will be the number corresponding to the other ray of the angle.
In the picture, since OB lies on 0, the measure of <AOB will be the measure corresponding to OA. Which is...?
This is written as m<AOB and said "the measure of angle AOB."
If one of the rays of the angle does not lie on 0, then the angle measure will be the difference in the number corresponding to the rays.