Letters Patent, or 'Litterae Patentes', represent the formal mechanism through which the Sovereign exercises the Royal Prerogative to create peerages, grant charters, or appoint high officials. Unlike 'Letters Close', which were sealed shut for private communication, Letters Patent are issued 'open' with the Great Seal pendant at the bottom, signifying they are addressed to all the King’s subjects as a public proclamation of the grantee's new status. In the context of the peerage, these documents define the specific grade of nobility—be it Duke, Marquess, Earl, Viscount, or Baron—and, crucially, the 'limitation' or line of descent by which the title shall pass to future heirs.
From a heraldic and legal perspective, the Patent serves as the definitive evidence of a peer's right to their seat in the House of Lords and their specific precedence. Since the 14th century, when creating peers by patent replaced the older method of creation by writ of summons, these documents have allowed the Crown to exert greater control over the exact terms of inheritance. The language is traditionally formulaic, originally written in Latin on vellum and decorated with illumination or the Royal Cypher, evolving into the standard English forms used by the Crown Office today.