Cardiac margins are clearly seen because there is contrast between the fluid density of the heart and adjacent air filled alveoli.

You cannot visualize the partition of the right and left ventricle because there is no contrast between them, both being of fluid density.

If the adjacent lung is devoid of air, the clarity of the silhouette will be lost.

Silhouette sign is extremely useful in localizing lung lesions.

Mediastinal and pleural disease should also be considered.