Just a quick demonstration of why there are two scales on a protractor. Many students don't seem to have a good conceptual grounding in how they should reason through which scale to use.
What angle degree is the same on both scales on the protractor?
Which scale is used to read the angle degrees before you reach 90 degrees when moving from left to right?
Which scale is used to read the angle degrees after you reach 90 degrees when moving from right to left?