Bruce Bueno De Mesquita, "Popes, kings, and endogenous institutions: The Concordat of Worms and the origins of sovereignty." International Studies Review (2000): 93–118. in JSTOR
One or more of the preceding sentences incorporates text from a publication now in the public domain: MacCaffrey, James (1908). "Pope Callistus II". In Herbermann, Charles (ed.). Catholic Encyclopedia. Vol. 3. New York: Robert Appleton Company.