Refers to the system of priority for patent applications formerly used in the United States. Under the first-to-invent system, the first person to reduce an invention to practice was entitled to any patent on the invention. The first-to-invent system led to a type of administrative patent litigation in the USPTO known as an interference, where parties disputed who invented first, who had the earliest reduction to practice and who should get the patent.

First-to-invent was usually defended in the United States as protecting the small inventor, who may not be sophisticated enough to file a patent quickly; however, it should be noted that it gave rise to the patent interference process, which could prove prohibitively expensive to a small inventor. The First-to-Invent system is in the process of being effectively ended by the America Invents Act, although it remains in place for patents applied for prior to 16 March, 2013 and thus in theory for a few years yet, though in practice the number of interferences is dwindling awaySee Priority Date, First-to-File.

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