The Circular Spline is a rigid ring with internal teeth, engaging the teeth of the Flexspline across the major axis of the Wave Generator. The Circular Spline has two more teeth than the Flexspline and is generally mounted onto housing.
The Flexspline is a non-rigid, thin cylindrical cup with external teeth. The bottom of Flexspline is called diaphram and generally mounted onto the output shaft.
The wave Generator is a thin raced ball bearing fitted onto an elliptical hub serving as a high efficiency torque converter and generally mounted onto the input shaft.