A goniometer is a device for measuring the range of motion of a joint.

The centre of the goniometer is placed on the jointline and the arms of the device point to hip and ankle.
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CLINICAL PUBLICATIONS
Reliability of goniometric measurements and visual estimates of knee range of motion obtained in a clinical setting. Watkins MA, Riddle DL, Lamb RL and Personius WJ. Phys Ther. 1991 Feb;71(2):90-6; discussion 96-7.