The elbow is the joint where the upper arm bone (humerus) meets the two bones of the lower arm (ulna and radius). Actually, it is a combination hinge and pivot joint. The hinge part of the joint lets the arm bend like the hinge of a door; the pivot part lets the lower arm twist and rotate. The rounded ends of the upper arm bone give the elbow its two "knobs" or bumps (epicondyle). Several muscles, nerves and tendons (connective tissues between muscles and bones) cross at the elbow.