This case illustrates the anatomy of the fissures of the right lung. The fissures can aid in localizing pathology to certain lobes. Usually, when right mid zone opacification is demarcated inferiorly, it is due to pathology in the upper lobe being limited by the horizontal fissure. In this case, a similar x-ray appearance has been caused by opacification in the lower lobe being limited by an accessory fissure called the superior accessory fissure.