Immobilization questions come from robotics, especially from grasping problems [5], [6]. In [1], a curvature criterion was established to decide when three points immobilize a plane figure. This curvature criterion proves that three points suffice to immobilize any convex figure, other than a disk, whose boundary has continuous curvature, and also that three points in the interior of the three sides of a triangle immobilize it if, and only if the three normal lines are concurrent (see also [2]).