 Building Blocks Using a protractor A protractor is used to measure the exact size of an angle in terms of the number of degrees between 0º and 180º. A protractor looks like: There are usually two sets of numbers on a protractor - one set running from left to right and the other set from right to left. These two sets of numbers make easier to line up the protractor on any angle, regardless of the direction of the angle. The halfway point of every protractor is 90º. It is only written once. Using the protractor To measure an angle with a protractor do the following: Place the circle at the bottom center of the protractor (or comparable vertex marker) dirctly on the vertex (point) of the angle. Carefully line up the black line at the bottom of the protractor (or comparable baseline indicator) along the bottom ray of the angle. Find the pair of numbers where the top ray of the angle intersects the degree markings on the protractor. Determine whether the angle is acute, obtuse, right, or straight. Decide which number fits this angle.