Letters patent
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
Open letters, as distinguished from letters close. An instrument proceeding from the government, and conveying a right, authority, or grant to an individual, as a patent for a tract of land, or for the exclusive right to make and sell a new invention. Familiarly termed a "patent."
See International Tooth Crown Co. v. Hanks Dental Ass'n (C. C.) Ill Fed. 918.