The radius (radial bone) is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally. The radius articulates with the capitulum of the humerus, the radial notch and the head of the ulna. The corresponding bone in the lower leg is the tibia.
The word radius is Latin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally from center of capitulum to the center of distal ulna.