An angle has a vertex that is the common end point of two rays.
To measure angle with a protractor place the center at the angle vertex,
one side along the bottom, and read the angle at the other ray.
The rays of the angle go through the holes on the beams.
Use a protractor to measure the angle the chair back makes with the seat.
The Chair project in the Simple Machine set can be set at 7 angles.
Measure the angles in the first six positions.
Now graph the data.
Use that information to estimate the reading for the seventh position.
Measure and see how close you came.