"The Letters Patent were ... written upon open sheets of parchment, with the Great Seal pendent at the bottom ... [while] the 'Litteræ Clausæ,' or Letters Close, ... being of a more private nature, and addressed to one or two individuals only, were closed or folded up and sealed on the outside." [S.R. Scargill-Bird, "A Guide to the Principal Classes of Documents at the Public Record Office," 1891]The adj. sense of "open to view, plain, clear" is first recorded 1508; the verb "to obtain right to land" is attested from 1675. The meaning "copyright an invention" is first recorded 1822, from earlier meaning "obtain exclusive right or monopoly" (1789), a privilege granted by the Crown via letters patent.