Letters patent were legal instruments in the form of open letters issued by a monarch or government for the purpose of granting a monopoly right or status to a person or to some entity such as a corporation. The term letter patent is derived from the Latin word, litterae patentes. Letters patent is an official document which gives someone the exclusive right to make and sell something which s/he has invented. Letters patent later evolved into the modern patent that grants exclusive rights for an invention.
Forgery of letters patent granted by the President is a crime which is subject to fine or imprisonment up to ten years.[Marbury v. Madison, 5 U.S. 137 (U.S. 1803)]
The following is an example of case law defining the term letters patent:
Letters patents is an open letter, and explains that "Letters patent are so called because they were commonly addressed by the sovereign to all subjects at large and were not sealed up like a secret commission, but open, ready to be shown to whom it might concern." [In re Bo Thuresson Af Ekenstam, 45 C.C.P.A. 1022, 1025 (C.C.P.A. 1958)]