Figure 1. A in Three-dimensional geometry of a pterosaur wing skeleton, and its implications for aerial and terrestrial locomotion

Figure 1. A pair of articulating elements with a single joint axis, set up so that the axis runs perpendicular to the viewing plane, and is perceived as a point – the centre of rotation (CR). The proximal element is fixed and has a single marker O, defining the origin of an arbitrary coordinate system. The distal element is mobile and bears two markers A and B, denoted A′ and B′ after a rotation of magnitude Q. The positions of the markers before and after angulation in the arbitrary coordinate system can be used to determine the position vector c of the CR, and the magnitude of rotation Q. See text for details.