--and you can even construct one with a piece of paper--is we've taken a half circle right here, and we've divided it into 180 sections, and each of these marks marks ten of those sections, and what you do for any given angle is you put one of the sides of the angle--so each of the rays of the angle are considered one of its sides--so you put the vertex of the angle at the center of this half circle, or if you're dealing with an actual protractor at the center of that protractor and then you put one side along the zero mark.