Omar, Hayley, Gary, and Autumn teach us how to use a protractor. They give a bit of information from About.com on the history of protractors. They are used to measure angles. The first protractor was used to help with navigation and invented in 1801. To measure an angle, line up the base of the protractor with 1 line of the angle. The center circle should be on the vertex of the angle. Use the "swivel thing" to find what degree the other end of the angle lines up with. If there isn't one, you can use a sheet of paper or ruler. You can also use it on a square or triangle. The video also shows how to find a missing angle of a triangle. If it says the degrees of the two other sides, add them up and subtract from 180. The resulting number is your missing side.