To use silouhette sign you need to know what structure are adjacent to each silouhette.

Silouhette	Adjacent lobe/segment
Right Diaphragm	RLL/Basal segments
Right Heart margin	RML/Medial segment
Ascending Aorta	RUL/Anterior segment
Aortic knob	LUL/Posterior segemnt
Left Heart margin	Lingula/Inferior segment
Descending Aorta	LLL/Superior and medial segments
Left Diaphragm	LLL/Basal segments

You should know that Pleura encircles lung and diseases of pleura can also can obliterate the silhouettes.

Same is true for mediastinal masses.

I will show you few examples.

Rt Heart Margin obliterated by RML pneumonia
Lt diaphragm obliterated by LLL pneumonia
Pleural effusion: Diaphragm and right heart margins are not detectable.
Posterior mediastinal mass: Aortic knob is indistict.