NORMAL CXR

In the outer zone, there should be virtually no lung markings visible.

In the middle zone, you can normally follow radiating pulmonary vascular markings for several generations of branching.  This is a good place to look for distinctness of vascular margins and distribution of flow relative to gravity.

The innermost zone is where the large pulmonary arteries are located.  This is the region to look for adenopathy and enlarged vessels due to pulmonary artery hypertension.  There is often a lot of overlap of structures in this zone.