A protractor is a tool for measuring angles, usually shaped like a half-circle with numbers marked along its curved edge from 0 to 180 degrees. To use one, you place the flat edge along one side of the angle and the center point (often marked with a small hole or crosshair) right at the corner where the two lines meet. Then you read the number where the other line crosses the curved edge: that's your angle measurement.

Protractors show up constantly in math class when you're learning geometry. You might use one to measure the angles in a triangle to confirm they add up to 180 degrees, or to draw a specific angle precisely for a construction project. Architects use protractors when drafting building plans, and navigators once used specialized protractors to plot courses on maps.

Most protractors are made of clear plastic so you can see the lines underneath, though wooden and metal versions exist too. Once you get comfortable with a protractor, you start seeing angles everywhere: in the corner of a book, the hands of a clock, or the slope of a roof.