> A good rule of thumb is if the derivative work is drawing sales away from the original work. That's a horrible rule of thumb. The canonical example of a deri

> A good rule of thumb is if the derivative work is drawing sales away from the original work. That's a horrible rule of thumb. The canonical example of a derivative work is adaptation--the film of the book, for example--which frequently increases sales of the original work. Drawing sales away from the original work is one of the factors used in fair use, and frequently ends up being the most important one.