From Moss & Adams(Vol. 1) 7th ed. 2008.
Chapter: Aortic Arch Anomalies by Dr. Paul Weinberg.
1) Arch sidedness: Retroesophageal or Isolated vessel always is the vessel on opposite side from the arch.
2) Vascular Ring: Easier to recognize the existence of vascular ring if all components of the ring are patent. The following rule is helpful when the ring is completed by an atretic ligament on one side i.e. Presence of one of the 3 Ds on the opposite side from the aortic arch indicates presence of a vascular ring. (3Ds are Diverticulum, Dimple or Descending aorta).