The plain film examination of the abdomen in decubitus position reveals a large amount of free air collecting in the right flank, clearly outlining the bowel wall (open arrows). When air is present on both sides of the bowel, the wall is outlined with clear distinction because of the contrast differences created on both sides. This is called Rigler's sign and is pathognomonic for free air in the peritoneal cavity. The yellow arrows show air-fluid levels in distended bowel.