The course of the azygos vein within the mediastinum determines the anatomic arrangement of the posterior right mediastinal pleura. Lung intruding into the mediastinal recesses above and below the azygos arch provides contrast which allows demonstration of detailed normal mediastinal anatomy. The roentgen appearances of these normal landmarks in frontal, lateral, and oblique positions are discussed and correlated with anatomic specimens. Knowledge of this anatomy is important to explain many features of the normal roentgen anatomy of the mediastinum and to prevent misinterpretation of these normal anatomic structures as mediastinal nodes or masses.