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In page Patent:

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Non-national treatments in national patent offices had been prevalent among the Northern countries[citation needed] until they were prohibited after the negotiation of the Paris Convention for the Protection of Industrial Property. According to Articles 2 and 3 of this treaty, juristic and natural persons who are either national of or domiciled in a state party to the Convention shall, as regards the protection of industrial property, enjoy in all the other countries of the Union, the advantages that their respective laws grant to nationals.