The Silhouette Sign can be used to determine what lobes of the lung are affected by an alveolar filling process. This can be demonstrated on chest x-ray when 2 structures of the same density touch, causing their borders to disappear. On x-ray, air is black while. soft tissues are white. Alveolar filling processes occur most commonly from pus as in pneumonia, edema, as in congestive heart failure, hemorrhage, or collapse. Pleural fluid can also produce a silhouette sign, recognized by blunting of the costophrenic angle or a meniscal sign.

The R lung has 3 lobes, the RUL, RML, and the RLL, which are separated by the horizontal and oblique fissures, respectively. The L lung has 2 lobes, the LUL and the LLL, seperated by the L oblique fissure. The Lingula is part of the LUL.

In pneumonia, when the silhouette sign obscures the R hemidiaphragm, the infiltrate is filling the RLL. When the right heart border is obscured, the infiltrative process is occuring in the RML. When the left hemidiaphragm is obscured, the pneumonia involves the LLL, and when the left heart border is obscured, the process involves the lingula.