There is no clear plane between the dura and arachnoid in situ. Instead of the virtual subdural space, there is a dura-arachnoid interface layer, which is structurally the weakest throughout the meninges. An extravasation of blood within the dural border layer splits it, leaving a few tiers of dural border cells over the arachnoid. These cells cover the internal surface of the hematoma, proliferate, and later on, form the inner membrane. The outer membrane is related to hematoma enlargement because of the repetitive hemorrhages whereas the inner membrane is related to liquefaction of the subdural hematoma. As the inner membrane plays a pivotal role in the pathophysiogenesis and determination of the location of chronic subdural hematoma, histologic, ultrastructural, and clinical analyses were performed with correlations to the dura-arachnoid interface and the so-called "subdural space."