The double wall sign is another name for the Rigler sign. It is a sign of pneumoperitoneum with air outlining both sides of the bowel wall. It is seen when large amount of free air, > 1000 ml, is present.
It should be distinguished from the false double-wall sign which refers to two dilated loops of bowel in close apposition such that the luminal gas of one loop appears to outline the serosal surface of the other loop. In the false double wall sign, the wall thickness is double that of a single loop.