CANONICAL Letters, in the ancient church, were a sort of testimonials of the orthodox faith, which the bishops and clergy sent each other to keep up the Catholic communion, and distinguish orthodox Christians from Arians and other heretics. They were denominated canonical, either as being composed according to a certain rule or form, or because they were given to the canonici, that is, those comprehended in the canon or catalogue of their church. When they had occasion to travel into other dioceses or countries, dimissory and recommendatory letters, also letters of peace, &c. were for many species of canonical letters.