Gripper and fixture grasps can fall under different categories. The two most important grasp classifications are known as form closure and force closure grasps. These terms were first used by Reuleaux in his studies of machine design, and were later applied to robotic grasping by Salisbury. However, recent inconsistent use of these terms has confused their meanings.
A fixed set of contacts on a rigid object is said to exhibit form closure if the object's equilibrium is maintained despite any possible externally applied wrench (i.e. a force, a torque, or a combination thereof). In other words, the geometries and relative poses of the object and the contacting body--a gripper or fixture, for example--suffice to hold the object in place with complete kinematic constraint. Form closure is therefore more preferable to force closure, in which the applied manipulator force is further required as a consideration.
In developing a gripper or fixture, one should therefore strive for form closure. In certain important instances, however, this would be is impossible. For example, any part which is axisymmetric (such as a sphere) cannot be kinematically constrained in form closure. Under such circumstances, a robotic manipulation strategy should strive for frictional form closure (a type of force closure) instead. Under frictional form closure, friction contributes to immobilize a grasped part which is otherwise not fully kinematically constrained by a gripper.