Apprentice Adept is a seven-book fantasy and science fiction novel series by 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 emigrated 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.