Letter Patents

Letters patent (always in the plural) are a type of legal instrument in the form of a published written order issued by a monarch, president, or other head of state, generally granting an office, right, monopoly, title, or status to a person or corporation. Letters patent can be used for the creation of corporations or government offices, or for the granting of city status or a coat of arms. Letters patent are issued for the appointment of representatives of the Crown, such as governors and governors-general of Commonwealth realms, as well as appointing a Royal Commission. In the United Kingdom they are also issued for the creation of peers of the realm. 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). 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 and also to understand how to "practice" the invention, i.e., put it into practical use.

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. Today, for example, it is a convention for the British prime minister to announce that they have left a document they wish to enter the public domain "in the library of the House of Commons", where it may be freely perused by all Members of Parliament.

...
Wikipedia