The forearm contains two long parallel bones: the radius and ulna. The radius is on the thumb side and is slightly shorter, but it forms the majority of the wrist joint distally. The radius pivots around the ulna at the wrist distally and the radial head rotates at the elbow joint proximally, which allows for forearm rotation. The forearm rotates approximately 90 degrees palm up (supination) and 90 degrees palm down (pronation). Compared to the radius, the ulna is the longer and larger of the two bones. It lies parallel to the radius on the small finger side of the arm.  At the elbow end of the ulna, also called the olecranon, it forms the hinge portion of the elbow joint with the humerus.
The forearm contains several tendons and muscles including the flexors and extensors and pronators and supinators.  There are two main fascial compartments in the forearm. The posterior (back) compartment contains the extensor muscles and tendons, which are innervated by the radial nerve. The anterior (front) compartment contains the flexors, which are innervated by the median nerve.  The ulnar nerve runs down the forearm on the small finger side.  Radial and ulnar arteries supply blood to the forearm and main superficial veins (the ones often used for blood draws) return it to the heart.