Sharp rib diaphragm angles bilaterally are normal. The diaphragm is a dome-shaped muscle that is high in the center and low on both sides, which causes the rib-diaphragm angle to show up as an acute state on x-ray. The diaphragm is a major respiratory muscle that closes the thoracic cavity (and also separates the thoracic and abdominal cavities), and the diaphragm keeps the thoracic cavity relatively closed (which facilitates the transition between expiration and inspiration). And there is a small amount of pleural fluid between the lungs and the thoracic cavity. When the pleural fluid is maintained at a normal amount, the rib-diaphragm angle is kept at an acute angle; when pleural fluid accumulates, the rib-diaphragm angle becomes larger or disappears.