The protractor is the instrument used to measure angles. The protractor has two scales running from 0 to 180 degrees in opposite directions. To measure an angle, place the center point of the protractor on the vertex, then align the zero on either side of the scale with one side of the angle. Since AG, in the interactive model, is aligned with the 0 on the inner scale, use the inner scale to find the measure of the angle GAF
Angles can be classified by their measures. A right angle has a measure of 90 An acute angle has a measure between 0 and 90. An obtuse angle has a measure between 90 and 180. A straight angle has a measure of 180.