↑The popes and the papacy in the early Middle Ages, 476-752 By Jeffrey Richards Routledge & Kegan Paul, 1979. ISBN0-7100-0098-7
↑Otto of Freising and his continuator Rahewin, The deeds of Frederick Barbarossa tr. Charles Christopher Mierow with Richard Emery. New York: Columbia University Press, 1953. Reprinted: Toronto: University of Toronto Press, 1994
↑History of the later Roman Empire from the death of Theodosius I to the death of Justinian by J.B. Bury (New York : Dover Publications, 1958) [1]