Hofflund, Ethel; Loeks Bouman, Elizabeth; Stitt, Howard; Christopherson, Alan (March 2001). "The Feudal System". In Rirchard W. Wheeler, M.A.Ed. (ed.). History & Geography 604 Life in the Middle Ages. 804 N. 2nd Ave. E., Rock Rapids, IA 51246-1759: Alpha Omega Publications, Inc. pp. 6–7. ISBN978-0-86717-554-7. Retrieved 19 March 2010.{{cite book}}: CS1 maint: location (link)
