On Friday, a federal circuit court made clear that Google Books is legal. A three-judge panel on the Second Circuit ruled decisively for the software giant against the Authors Guild, a professional group of published writers which had alleged Google’s scanning of library books and displaying of free “snippets” online violated its members’s copyright.

To some digital-rights followers, the Google Books case had seemed to drag on forever: The Authors Guild first filed suit 10 years ago. But the theory behind the eventual ruling was a quarter-century in the making.
In 1990, a district-court judge named Pierre Leval published an article in theHarvard Law Review proposing a new theory of fair use. In fact, the article was named “Toward a Fair Use Standard.” (The Harvard Law Review, by the way, had one month earlier elected its first black president.)

 Fair use—which lets people use and adapt copyrighted works without getting the explicit permission of their owner—is a distinctly American concept. Instead of setting out specific statutory exemptions to copyright, as many other countries do, U.S. law issues four broad factors which guide whether the permission-less use of a copyrighted work is fair. This means that fair use can evolve and change over time; it also means that the only real way to find out if something is “fair use” is to ask a federal court.