Evaluation of coccygeal mobility. A, Standing radiograph. B, Sitting radiograph shows flexion of the coccyx. C, Superimposition of sitting radiograph on standing radiograph, matching the sacrum by pivoting the sitting film through an angle representing sagittal pelvic rotation (angle 1= angle of rotation). Coccygeal mobility is indicated by angle 2 (angle of mobility). Angle 3 is the angle at which the coccyx strikes the seat surface (angle of incidence).