The most common form of vascular ring compression is PRAA with absence of the left arch.  The intersegmentals are pulled forward along the aorta as usual but the left arch instead of the right arch separates from the dorsal aorta.  Consequently, the right 4th aortic arch,  left-sided ductus arteriosus and pulmonary artery form a vascular ring around the esophagus and trachea.  The esophageal compression is relieved by dividing the ductus (or ligamentum arteriosum).