I noticed right away the claim that Charlemagne is the 27th Great-Grandfather of Prince William. The chart of this ancestral line is here on Geni.com.
I went back to the Ancestors of Prince William site and noted that on the list there that Charlemagne is listed as a 31st Great-Grandfather (in ten places!). There is one listing for him as a 30th great-grandfather, but it is as an Alternate sperm donor, er, ancestor.
So which one is right? I don't know. I started to compare the two lists. The first difference I saw was in the 13th generation back from William - Geni.com has the mother of Christina von Hessen as Christine of Saxony; the FabPedigree site has the mother as Margaret von der Saale (1522-1566), and lists Christine Wettin of Saxony (1505-1549) as an alternate. The trail back on the FabPedigree site is lost after one more generation, wh. ile the Geni.com database shows the line back to Charlemagne.
What are the authoritative sources used by these databases? Jamie D. Allen, who owns the FabPedigree site, lists his sources and contributors - many of whom are fairly well known and active in medieval genealogy circles. As far as I can tell, Geni.com doesn't list the sources for Prince William's tree.
An anonymous comment on my earlier post suggested two websites -
1) www.Genealogics.org - nice site, Prince William's page is here. Unfortunately, I can see only 8 generations at a time, and can't quickly look at the line back 30-some generations.
2) Paul Theroff's Online Gotha. The Great Britain page has only descendants of George I, King of England.
I am drawn to this "celebrity ancestor" part of genealogy like a magnet, it seems. Must.not.submit.to.this.folly! Or I could add several more generations to my Geni.com tree and try to show off my own illustrious royal ancestry. Tough choice! I'll get back to you on this...