Only two actions are possible in the radio-ulnar joint: supination and pronation. In supination the forearm is rotated so that the hand is turned palm up. In pronation the forearm is rotated so that the hand is turned palm down. At the elbow the radius rotates around the annular ligament and does not change positions relative to the ulna. At the wrists, when the forearm is in pronation, the radius crosses over the ulna so that it is then on the inner side of the ulna. When the forearm is in supination the radius is on the lateral side of the ulna.
