I got this in my e-mail and thought it was interesting:

undecimal (UHN-des-uh-muhl) adjective
Based on the number eleven.
[From Latin undecim (eleven).]

Some people do have undecimal hands: they might have eleven fingers. The bonus digit is called supernumerary. Supernumerary parts don't have to be fingers. For example, instead of a binary arrangement, some people are gifted with an extra nipple. You could call them "supernumammary".

-Anu Garg
gargATwordsmith.org

"A modern though little realised example of undecimal counting is seen in the ISBN of published books. Any ISBN comprises ten digits. If you multiply the first by ten, the second by nine, the third by eight, and so on, summing the results as you go along, the result will always be divisible by eleven."

William Hartston; What Are The Chances Of That?: Fabulous Facts About Figures; Metro Books; 2004.

(Try this on the ISBN of my new book: 0471718459. -Anu)
Anu Garg's new book:
Another Word A Day : An All-New Romp through Some of the Most Unusual and Intriguing Words in English Publisher: John Wiley & Sons