Diadem is a series of fantasy novels by John Peel. The series takes place on several different worlds, collectively referred to as the Diadem. Travel between worlds is accomplished through the use of magic portals. The physical distance between worlds is irrelevant. Portal travel is based on the mystical geography of the Diadem, which is divided into five layers. The layers are a little like the layers of an onion and a drawing of them is the logo of the Diadem series and is on every book. At the center is a single world called Jewel, followed by the Inner Circuit, Middle Circuit, Outer Circuit, and Outer Rim. Magic is strongest on Jewel, and progressively weaker the "farther" a world is from the center. In the Outer Rim, magic barely functions at all.