Fair use might seem like an esoteric legal concept, but it’s what lets scholars cite copyrighted novels in their research—and what lets Big Dog sell movie-parody t-shirts. It’s also what lets me drop an image of one of those shirts into this story:

Big Dog

In his article, Judge Leval argued that the degree to which some kind of use transformed the original work should principally guide fair-use rulings. Fair use “must employ the quoted matter in a different manner or for a different purpose from the original,” he wrote:

A quotation of copyrighted material that merely repackages or republishes the original is unlikely to pass the test; in Justice Story’s words, it would merely “supersede the objects” of the original. If, on the other hand, the quoted matter is used as raw material, transformed in the creation of new information, new aesthetics, new insights and understandings, this is the very type of activity that the fair use doctrine intends to protect for the enrichment of society.

Leval then lists some example fair uses:

Transformative uses may include criticizing the quoted work, exposing the character of the original author, proving a fact, or summarizing an idea argued in the original in order to defend or rebut it. They also may include parody, symbolism, aesthetic declarations, and innumerable other uses.

“It’s a famous passage,” says James Grimmelmann, a law professor at the University of Maryland and the director of that school’s intellectual-property program. Leval had “written a couple of decisions that dealt with authors whose works engaged with existing ones and necessarily had to quote a lot. He put together an idea that wove together some major themes in the cases: Fair use protects people who make transformative uses of existing works.”

Four years later, the Supreme Court cited Leval’s article in the “2 Live Crew” case, which ruled that that rap group’s parody of “Pretty Woman”—which sampled Roy Orbison’s original—was a fair use because it was a parody.

“Based upon that, this idea of transformative use as the keystone to a lot of fair-use cases becomes dominant in U.S. courts,” Grimmelmann told me. “And what happens then over the next two decades is that the idea of transformative fair use itself gets extended to all kinds of new situations, including search engines.”

A series of decisions in the Ninth Circuit—the federal court of appeals that includes California, and, thus, Silicon Valley—found that software that searches web pages and images were so transformative as to be fair use. Showing a thumbnail image to someone to help them decide whether to go to a webpage, for instance, wasn’t the same as showing an image to entertain or inform.

This and other decisions, says Grimmelmann, were “very controversial because people say its abusing the concept of transformative use.”