A closely-watched patent case pitting Microsoft against a Canadian software firm is being argued today before the Supreme Court. If Microsoft wins it could overturn decades of patent law.

i4i is a Canadian software firm that filed suit against Microsoft in 2007. The center of the case was a patent i4i had filed for separating the formatting from text in a word processor. The patent was filed in 1994.

Microsoft asked the Patent and Trademark Office to re-examine the patent. The first time, the PTO said i4i's patent was valid. Microsoft asked a second time and the PTO declined to review the patent at all.

Microsoft argues that the patent is invalid because a similar technology already existed - a situation known in patent law as 'prior art.' Microsoft also asked for the standard of proof for a patent's validity to be lowered from clear and convincing to preponderance of evidence. In the courts, the former is a much tougher bar to clear.

i4i fired back with its brief in March, which says changing the standard of evidence would undermine the presumption that a patent is valid when it is challenged. In addition, i4i says the clear and convincing standard is codified in law and has been since the 1950s.

Meanwhile, i4i won its case in the Federal District Courts, and Microsoft was ordered to pay $290 million. Microsoft eventually asked the Supreme Court to hear the case.

Microsoft has thus far been supported by a number of technology firms including Google, Hewlett-Packard, Dell and even Wal-Mart. Among i4i's backers are the biotechnology industry, the U.S. Solicitor General and companies such as Dolby Laboratories and 3M.