Elbow - Lateral or Side View
The elbow, or elbow joint, is formed by the connection of three bones: the humerus in the upper arm, and the radius and ulna in the lower arm. The bony point of the elbow, that most people think of as the elbow, is actually the tip the ulna bone.
The humerus, radius and ulna are held together through a series of muscles or ligaments. These muscles allow the elbow to act like a hinge, so the arm can be bent and straightened. This 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. With help from the wrist, 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 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.
You often need to combine flexing and rotating to move your arm in complex ways. For example, you need to both flex and rotate your arm when you pick up food and put it in your mouth. Try it and watch how your arm moves.