The elbow joint is a compound joint. Functionally, it consists of three partial joints with a common joint capsule. This is where the upper arm bone and the two forearm bones (radius and ulna) each join together with one of the two other bones. Through the elbow joint, the forearm can be bent or extended with relation to the upper arm. In addition, the joint functions in the twisting movements of the hand, called pronation and supination. These movements are in part made possible by a complex twisting movement of the radius.