This is an amateur version of a medical device called a goniometer, which can be used to measure range of motion in joints. My daughter broke both bones in her lower leg (tibia and fibula) and wanted to measure the progress she was making in physical therapy with the range of motion (flexing) in her knee. It was difficult to measure the angle of the knee joint with a standard protractor. The midpoint of the circle on the goniometer is placed in the middle of the knee joint on the outside of the leg, and the arms are pointed toward the ankle and the hip. A "good range of motion" in a normal patient is around a 120-130 degree angle, according to the physical therapist. Print yours and measure away! You will need the following hardware: three 3 mm x 15 mm long bolts one 3 mm elastomeric stop nut two 3 mm regular nuts print one of each of the three.stl files align the stationary.stl with two outer holes in the protractor and glue together with acetone. temporarily bolt together to maintain alignment while the acetone is drying.. TO USE: Align the device on the side of the knee or joint with the center bolt at the pivot point of the joint. hold the stationary piece and the pointer to opposite sides of the joint. the angle of movement can be read from the device.