When the ancient Christians were about to travel into a foreign country, they took with them letters of credence from their own bishop, in order that they might communicate with another church. These letters were of three kinds: epistole commendatoriae, given to persons of quality, or persons whose reputation had been called in question, or to the clergy who had occasion to travel into foreign countries; epistola communicatoriae, given to such as were in peace and communion with the Church; epistolae dimissoriae, such as were given by the bishops to the clergy when removing from one diocese to another. All these were called epistolae formatae, because they were written in a peculiar form, with certain marks, which served to distinguish them from counterfeits. Farrar, Ecclesiastes,Dictionary, s.v.; Bingham, Orig. Eccl. book 2, chapter 4.