The US Supreme Court immersed itself in patent rules in a case pitting Microsoft against a Canadian technology company that claims some versions of Word violated its patent.
Eight of the nine judges -- the chief justice recused himself -- questioned the companies' lawyers on Monday at length on whether rules for challenging the validity of patents should be changed.
Beyond the battle between the US software giant and Toronto-based i4i, at stake is the future of distribution of patents in the United States.
A Microsoft victory would make it harder for patent holders to prove infringements. But if the court sides with i4i, some say it would stifle innovation.
"It's a bad thing not to give protection to an invention that deserves it; and it is just as bad a thing to give protection to an invention that doesn't deserve it," justice Stephen Breyer said. "Both can seriously harm the economy."
Microsoft wants those accused in infringement cases to be able to prove a patent invalid by showing a "preponderance of evidence," rather than the tougher standard of "clear and convincing evidence" that it was held to in this case.
In 2007, a lower court ordered Microsoft to pay $290 million to i4i for patent infringement related to the Canadian company's rights to XML technology.
A decision is expected by the end of June.