Double track system
If you are sued by a patentee for patent infringement, one of the options you often have would be to attempt to invalidate the patent on the ground that the patent lacks patentability. The issue here is whether you can assert invalidity of the patent at issue to an infringement court, or you need to initiate a separate proceeding at a national patent office or a specialized patent court in order to argue patent invalidity. As for Japan, in addition to an invalidation proceeding at the Japanese Patent Office (Track 1), Japanese patent law allows a defendant in an infringement court to assert patent invalidity within the same proceeding (invalidity defense), and the court can judge the validity on its own (Track 2). This legal system can be referred to as a “double track system”. Examples of jurisdictions with this system besides Japan are the U.S., the U.K., France and Korea.