Pneumothorax chest x ray

Overview
In a supine chest X-ray the deep sulcus sign is diagnostic, which is characterized by a low lateral costophrenic angle on the affected side. In layman's terms, the place where rib and diaphragm meet appears lower on an X-ray with a deep sulcus sign and suggests the diagnosis of pneumothorax. Images shown below are courtesy of RadsWiki and copylefted