The Wold Newton family is a literary concept derived from a form of crossover fiction developed by the science fiction writer Philip José Farmer. He links, through a common origin, the lives of real fictionalized characters, both heroic and villainous such as Sherlock Holmes, Tarzan, Doc Savage, and Lord Peter Wimsey.

Other examples of this are Kim Newman's work. Warren Eli's Planetary, Alan Moore's League of Extraordinary Gentlemen.

Wold Newton fictions can very easily be secret histories when they purport to give a secret accounting of what has happened in the world. A truer history. At other times they stray into alternate history land.

Can anyone think of any other ones I'm missing? What are your favorite elements of a Wold Newton?