A semicircular tool for the measurement and construction of angles. On the flat side of the protractor in the middle is either a hole or cross hairs. This is where the vertex (point of the angle) must go when measuring. Usually a horizontal line goes through this point and from one side of the protractor to the other. Sometimes there are no numbers on this line, but 0? or 180? are measured from here. Along the curved part of the protractor there are number markings (also called calibrations), with 90? in the middle. Most protractors have two sets of numbers for each marking so you can measure an angle from either side. One set ascends from 0? to 180? (left to right) and the other set descends from 180? to 0? (right to left). A full circle has 360?. When you line up the protractor along one side of an angle, it will be lined up at 0? (on either the inner or outer set of numbers). This will tell you which set of numbers to read.To measure an angle, put the vertex of the angle at the hole or cross hairs, then turn the protractor so one side of the angle aligns with a zero point. Check to see that the 0? is among the inner or outer sets of numbers, then note the position of the other side of the angle. Where it crosses the curve of the protractor determines the number of degrees (?) of the angle. If your angle has more than 180?, mark the 180? position and measure the additional degrees from there.