Holding the transducer longitudinally at the level of the diaphragm allows interrogation of the lung superiorly, the costophrenic angle and the area just below the diaphragm.
This is a very good place to look for small pleural effusions as they are dependent and found in this location.
The chest wall can be plainly seen with subcutaneous tissue and then thoracic muscle (the intercostals).
The lung is seen as a bright curtain that descends with each inspiration. Normal lung is aerated and no fluid is seen in the costophrenic angle. As one cannot interrogate deep to air the lung hides the diaphragm as it domes up deep to the lowest angle of the lung. In this case the spleen is seen below the diaphragm.
Where there is a pleural effusion this fills the angle, pushes lung out of the way and lets the sonologist see through to view the diaphragm and underlying structures.
From this view the thickness of the chest wall, and the size and character of any pleural effusion can be assessed.