The patient can also be examined in a lateral decubitus position. This could be helpful to assess the volume of pleural effusion and demonstrate whether a pleural effusion is mobile or loculated. You could also look at the nondependent hemithorax to confirm a pneumothorax in a patient who could not be examined erect.  Additionally, the dependant lung should increase in density due to atelectasis from the weight of the mediastinum putting pressure on it.  Failure to do so indicates air trapping.