Alice v. CLS Bank
The 2014 US Supreme Court ruling that an abstract idea does not become patentable just because it runs on a computer.
Alice Corporation held patents on a computerised system in which a third party ensures both sides of a financial transaction meet their obligations — at heart, a very old business arrangement. CLS Bank sued to have the patents declared invalid. The Court was unanimous, and Justice Clarence Thomas wrote the opinion in June 2014, applying a two-step test from the Mayo case two years earlier: is the patent directed at an abstract idea, and if so, does it add anything inventive beyond a generic computer?
The answer was no, and the effect was large. Thousands of software patents have since been struck down on Alice grounds, and patent trolls found their business model noticeably more expensive. What actually counts as "abstract", however, the ruling never defined, so lower courts have spent a decade guessing.