Apprentice Adept is a heptalogy of fantasy and science fiction novels written by English American author Piers Anthony. The series takes place on Phaze and Proton, two worlds occupying the same space in two different dimensional planes. Phaze is a lush planet of magic, where Proton is a barren mining planet of science. As the series opens, each person born on Phaze and Proton has an alternate self living on the other world. But if a person on either world lacks a duplicate (for instance if a Proton citizen immigrated there from another planet, or a counterpart from the opposite frame died), he can cross to the other through an energy "curtain" that circumscribes each frame. The first three books in the series follow Proton serf Stile as he enters Phaze and becomes an important political force there. The next three concern the adventures of Mach (Citizen Blue's son), Bane (Stile's son), and Bane's companions. Finally, volume 7, Phaze Doubt follows Bane's and Mach's nine-year-old children, Flach and Nepe, among others.