Gene W. Heck When worlds collide: exploring the ideological and political foundations of the clash of civilizations Rowman & Littlefield, 2007 ISBN 0-7425-5856-8, p. 172 Google Books Search
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Ruhli, F.J, Blumich B, Henneberg M. (2010). Charlemagne was very tall, but not robust. Economics and Human Biology 8 289 – 290. DOI:10.1016/j.ehb.2009.12.005