IN 1572 the Privy Council of Elizabeth I, the queen of England, refused to grant patent protection to new knives with bone handles because the improvement was marginal. It is only natural that things progress, the council reasoned; minor ameliorations do not cut it. This week America's Supreme Court decided likewise.

Ruling on KSR International v Teleflex, a patent dispute centred on the addition of electronic sensors to car-accelerator pedals, the court said that the combination of two existing technologies was not sufficiently “non-obvious” to deserve a patent. “Granting patent protection to advances that would occur in the ordinary course without real innovation retards progress,” wrote Justice Anthony Kennedy in his opinion for the court. To obtain a patent's 20-year exclusivity, an invention is expected to be novel, useful and non-obvious—but the third requirement has not been rigorously applied in recent years by the patent office and the courts. Now examiners and the courts have more discretion to use “common sense”.

The ruling has sweeping implications. “Nearly every patent in force today is prospectively open to challenge,” says Bruce Lehman, a former commissioner of the United States Patent and Trademark Office who works at Akin Gump, a law firm. He expects a huge increase in litigation, longer waits to get patents and ultimately less certainty over their legitimacy. Yet this is beneficial, believes Brian Kahin, an intellectual-property expert at the Computer & Communications Industry Association, an industry lobby, since it may reduce the number of trivial or dubious patents.

The computer industry welcomes the ruling as a way to thwart the growing number of frivolous lawsuits by “patent trolls”—firms that make a business of suing others for violating questionable patents. But it is a setback for the drug industry, which often seeks new patents for minor tweaks to existing inventions, such as combining one drug with another.

The ruling is just one in a string of recent cases in which the Supreme Court has sought to reverse the trend towards making patents easier to obtain and enforce. Last year in a case involving eBay, the biggest online auction site, the court tightened the standards that determine when an injunction can be used to force a firm accused of patent infringement to stop trading. In January the court ruled in a dispute between two biotech firms that companies which license a patent from its owner may still challenge its legality.

And this week in a separate decision the court ruled in favour of Microsoft, the world's biggest software company, and against AT&T, the world's biggest telecoms firm, in a dispute over damages for patent infringement. It ruled that such damages should be limited to sales in America, but not abroad. This will help Microsoft in another infringement case in which it was ordered to pay $1.5 billion to Alcatel-Lucent, a telecoms-equipment firm, largely on the basis of sales outside America.

The underlying problem is that as the number of patents, and the value of each one, has increased tremendously, the system has been slow to adapt (see chart). The flood of applications taxed patent offices, creating huge backlogs and lengthy delays. Standards slipped. The number of lawsuits and value of settlements shot up. Attempts over many years by Congress to reform the system stalled, owing to a lack of agreement between the computer and drug industries on what should change. So the Supreme Court's decisions try to do what policy-makers could not.

How non-obvious an idea needs to be to qualify for a patent has long vexed America's legal minds. The invention had to be “something more than the work of a skilled mechanic,” the Supreme Court opined in 1850. In 1941 it set the bar higher, requiring a “flash of genius”. In 1952 Congress loosened the standard, stating that the idea simply needed not to be obvious “to a person having ordinary skills”.

This week's ruling provides the contours of a modern patent policy, by implicitly stating that inventors ought to be familiar with practices from other fields and that combining existing technologies is not enough, says Dominique Guellec of the Organisation for Economic Co-operation and Development. It may thus end the boom in reviled “business-method patents”, which often entail the application of obvious things, such as shopping or auctions, to an online setting. And it will probably prompt patent regimes in other countries to become more stringent, too.