System used to describe congenital heart malformations. Type 1, truncus arteriosus with 1 arterial trunk giving off aorta and main pulmonary artery; type 2, right pulmonary arteries originating acdjacent to each other off dorsal wall of truncus; type 3, right and left pulmonary arteries originating from either side of truncus; type 4, proximal pulmonary arteries missing.