The lower arm consists of two bones extending from the elbow to the wrist, running parallel to each other. The radius is on the outside, or lateral side, of the elbow. It connects to the thumb side of the wrist. The radius is bigger and longer than the ulna which is on the inside, or medial side, of the forearm closest to the body. The bony point of the elbow, that most people think of as the elbow, is actually the tip the ulna bone.
The radius and ulna connect to the humerus bone of the upper arm at the elbow joint. Thehe elbow joint consists of a series of muscles or ligaments acting like a hinge so you can bend and straighten your arm. This is bending and straighten is called flexing and extending the arm, respectively.
The elbow also provides for rotation of the lower arm through twisting of the radius and ulna. This twisting allows you to turn your hand palm up or palm down. The rotation of the lower arm is called pronation or supination depending on the direction of movement. Normally, the radius and ulna are parallel to each other. During pronation, the radius rolls around the ulna at both the wrist and the elbow. In this position, the radius and ulna appear crossed.
Here is an experiment. You can actually feel the radius and ulna rotating. Hold your right forearm with your left hand. Then rotate your right wrist palm up and then palm down. You will feel the radius and ulna bones in your right arm twisting and rotating.