The x-ray film is made by projecting x-rays through organs or structures of the body onto photographic film. Because some tissue allows less x-rays to pass through than other tissue, a shadow is created on the film that is the image of the radiopaque substance. Dense structures, such as bone, allow few x-rays to pass through, so the film is only slightly exposed; such structures appear white on the film. Hollow structures, such as the lungs, allow most of the radiation to pass through, almost fully exposing the film; such structures appear black on the image. For a chest x-ray, the subject stands between the x-ray machine and the photographic film, and is asked to take a deep breath and hold it until the procedure is complete.