The Wilmer Rotational Instant Seizing Tentacle (WRIST) is a Voluntary Closing (VC) hook prosthesis for transradial amputees, developed by the WILMER Group at the Delft University of Technology in the Netherlands. Dissatisfaction with the currently used bodypowered arm prostheses, expressed by patients and members of the WILMER group, led to a study devoted to the design of a new kind of VC arm prosthesis. This research resulted in a prototype of the WRIST which is neither elbow nor shoulder controlled. The WRIST consists of a two-fingered hook connected to a socket by a hinge. The distance between the hook fingers is related to the manually adjustable hook-socket angle in a way that dorsal flexion of the hook results in grasping. This provides the concept with some unique features: any pinching force can be locked to prevent fatigue, the hook can be closed at all times, and the system does not require a harness or operating cables. This improves cosmetics and comfort as well as functionality.