From the introduction: Oblivion is the realm of the Daedra, containing many different planes, each associated with a Daedric Prince. Each plane of Oblivion is entirely different, being a part of its prince’s personality. The sixteen main planes of each prince are accompanied by thousands upon thousands of other demiplanes and pocket dimensions. These smaller planes are rarely visited, and almost never governed by a being. The sixteen planes of the Princes are incredibly large, often being the size of entire worlds. These planes are inhabited by lesser Daedra specific to the plane they are on.