The elbow is comprised of three bones: the humerus (upper arm bone), and the ulna and radius (the two bones of the forearm). Three joints link these bones: the humeroulnar, humeroradial, and radioulnar joints.
The humerus articulates with both bones in the forearm, the ulna and radius. The two joints linking these bones—the humeroulnar and humeroradial joints—are hinge joints and are capable of flexion and extension of the elbow.
Flexion involves the movement of the hand and forearm toward the shoulder via rotation around the joint. Extension at the humeroulnar joint is the opposite of flexion and is the movement of the hand and forearm away from the shoulder. As one can see in the illustration, the position of the arm at the shoulder changes what the movement looks like (while the previous definitions of the movements hold).
The third joint active at the elbow is that between the two bones of the forearm, the radius and ulna. The articulation of the two bones—the radioulnar joint—rotates in two directions and facilitates pronation and supination of the forearm and hand. Pronation is the inward rotation of the forearm and hand, locating the palm of the hand down (thumb toward the body). Supination is the outward rotation of the forearm and hand, locating the palm up (thumb away from the body).