A dead letter a law or practice no longer observed. Originally with reference to biblical passages in which St Paul compares the life-giving spirit of the New Testament with what he sees as the dead ‘letter’ of the Mosaic Law. Also, until the late 19th century, the Dead-letter Office was the name given to the organization dealing with unclaimed and misdirected mail.
letter of marque a licence to fit out an armed vessel and use it in the capture of enemy merchant shipping and to commit acts which would otherwise have constituted piracy.
letters patent an open document issued by a monarch or government conferring a patent or other right.
See also red-letter day, scarlet letter.