goniometer

A goniometer is an instrument for measuring angles or the angular separation between two lines or parts. It is used in many fields to quantify orientation and movement of objects or structures. The most common forms are mechanical goniometers, often shaped as a circular or semicircular protractor with a movable arm, and electronic or digital goniometers that provide a readout of angle measurements.

In medicine and physical therapy, a goniometer (often a manual two-arm device) is used to measure a patient’s joint range of motion. The fixed arm aligns with a proximal segment, while the moving arm aligns with the distal segment; the included angle is read in degrees. These devices are standard in assessing knee, elbow, shoulder, and hip motion, with clinician training to ensure consistent landmark placement. Variability can occur between observers, and typical precision is around ±1–2 degrees under good technique.

In crystallography and materials science, a goniometer is part of X-ray diffractometers and goniometric goniostats that rotate the crystal, the sample stage, or the detector around precise axes to determine lattice parameters and crystal orientation. Four-circle and kappa-type goniometers are examples that enable complex angular positioning.

Other uses include optical alignment and surveying, where goniometers measure angles between components or reference lines. The term derives from Greek gonia, meaning angle.