Figure 2 in Three-dimensional geometry of a pterosaur wing skeleton, and its implications for aerial and terrestrial locomotion

Figure 2. Diagrammatic representation of an articulating element undergoing a cardinal angulation (A) and an arcuate angulation (B). During a cardinal angulation, the joint axis remains fixed and the moving bone remains in a single plane. During an arcuate angulation, the joint axis rotates and the bone moves out of the plane.