It is classically taught that mediastinal air enters the abdomen posteriorly by dissection along vascular sheaths. Present studies in infants, children, and young adults demonstrate dissection of extrapleural air to the subdiaphragmatic extraperitoneal space between the sternocostal origins of the diaphragm. The presence of extrapleural air above and extraperitoneal air below the diaphragm clearly defines the anterior diaphragmatic attachments, producing a characteristic radiographic sign. Recognition of this sign permits precise localization of air collections above and below the diaphragm, implying subdiaphragmatic extension of extraalveolar air rather than retroperitoneal perforation of a hollow viscus. Since collections of both extrapleural and intrapleural air may collect in a subpulmonic location, the ability to distinguish one from another has definite implications regarding chest tube placement.