Letters patent are a type of legal instrument in the form of a published written order issued by a monarch or president, generally granting an office, right, monopoly, title, or status to a person or corporation. They are so named from the Latin verb pateo, to lie open, exposed, accessible. The originator's seal was attached pendent from the document, so that it did not have to be broken in order for the document to be read.
They are called "letters" (plural) from their Latin name litterae patentes, used by medieval and later scribes when the documents were written in Latin, in the ancient sense of a collection of letters of the alphabet arranged to be read rather than in the modern sense of an "epistle" or item of correspondence: thus no singular form exists.
Letters patent can be used for the creation of corporations or government offices, or for the granting of city status or a coats of arms. A particular form of letters patent has evolved into the modern patent (referred to as a utility patent or design patent in United States patent law) granting exclusive rights in an invention (or a design in the case of a design patent). Clearly in this case it is essential that the written grant should be in the form of a public document so other inventors can consult it to avoid infringement.
The opposite of letters patent are letters close (Latin: litterae clausae), which are personal in nature and sealed so that only the recipient can read their contents. Letters patent are thus comparable to other kinds of open letter in that their audience is wide. It is not clear how the contents of letters patent became widely published before collection by the addressee, for example whether they were left after sealing by the king for inspection during a certain period by courtiers in a royal palace, who would disseminate the contents back to the gentry in the shires through normal conversation and social intercourse. Clearly some such mechanism was essential. Today, for example, it is a convention for the British prime minister to announce that he has left a document he wishes to enter the public domain "in the library of the House of Commons", where it may be freely perused by all MPs.