The defense is “a last-ditch safety valve” for products that technically fall within a patent’s claims but are so changed from the patented technology that treating them as infringing would be unfair, said Mark Lemley, a Stanford Law professor. He imagined a scenario in which Boeing developed the 747 just a decade after the Wright brothers first flew.

“The 747 has a flexible wing and a controllable rudder, so it would literally infringe the Wright patent,” Lemley said, “but it wouldn’t make sense to say the Wrights got to control that new technology.”