An azygous fissure is seen on approximately 1-2% of chest x-rays. In fact autopsy studies suggest they are much more common.
The azygous vein, lined by visceral and parietal pleura lies in the base of the fissure, and medially there is a small connection of the 'azygous lobe' with the rest of the upper lobe.
An azygous fissure has a tadpole shape, the azygous vein itself forming the head of the tadpole. Because the azygous vein is a right sided structure, this accessory fissure is only ever seen on the right side of the chest.