This mnemonic is commonly used to recall the differential diagnosis of a unilateral hyperlucent hemithorax:
C – contralateral lung has increased density or opacity (such as posterior layering of a pleural effusion in the supine patient)
R – rotation of the patient causing less attenuation of the x-ray beam
A – air (pneumothorax)
W – wall (chest wall, poland syndrome, mastectomy)
L – lungs (airway obstruction, emphysema, swyer-james syndrome, large bullae, large embolus)
S – scoliosis
Read about the differential diagnosis of a unilateral opaque hemithorax (or “white out”) here.