Pseudo-Isidore informs us that it was stated at the Council of Chalcedon by Atticus, bishop of Constantinople, that it was agreed at the Council of Nice that all such letters should be marked Π. Υ. Α. Π. (i.e. Father, Son, Holy Spirit), and it is asserted (Herzog, Real-Encyk., s.v. Literæ Formatæ) that this form is found in German documents of the sixth century.