Vigesimal: Pertaining to a base-20 numeral system. Pronunciation: vī-ˈje-sə-məl. From Latin vicesimus or vigesimus: "twentieth."
In Biting the Wax Tadpole: Confessions of a Language Fanatic, a delightful (and delightfully illustrated) little tour of the world of languages, author Elizabeth Little observes:
Vigesimal systems (base-20) are not at all uncommon--a fact that isn't remotely surprising if you've ever bothered to add your fingers to your toes. The Maya, accomplished mathematicians and astronomers, are one of many people to rely on a vigesimal system: in Mayan, the number thirty-one is buluc tu-kal--literally, "eleven after the twentieth."
Little also writes about a traditional vigesimal counting jargon once used throughout West Britain, primarily for counting sheep:
The words themselves are utterly charming, sounding like nothing so much as the names a young Will Shakespeare might have conjured up for a litter of adorable kittens: yan, tan, tether, mether, pip, azer, sezar, akker, conter, dick, yanadick, tanadick, tetheradick, metheradick, bumfit, yanabum, tanabum, tetherabum, metherabum, jigger.
The title of Little's book comes from the story that the original Chinese transliteration of "Coca-Cola," back in 1928, was ke-kou ke-la--literally, "Bite the Wax Tadpole." According to Snopes.com, fearless debunkers of urban legends, that's not quite right. The final agreed-upon transliteration translates to something like "to allow the mouth to be able to rejoice."
Bite The Wax Tadpole is also the name of an Irish web development company; an amusing explanation of the name choice appears here.