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I uncovered this word while researching why it is that Germanic languages use singular words to 12 and thereafter add the teen for ten. I am not entirely convinced by the range of ideas, such as Proto german started as a base 12 system due to having three joints in each of the four fingers or that this is why there are 12 inches in a foot, 12 in a dozen, 12 months in a year. I suppose you could also include the clock. At any rate a base 12 system is a duodecimal system, that is, two plus ten, I guess. From the Latin for 12.

This was the answer to a question at pub trivia this Thursday. Pretty much everyone knew it, though, so it wasn't much of a poser.

And then there's the "Duodecim Dies Festali", sung every year at the Latin Carol Celebration. Recorded here for your enjoyment:

https://www.youtube.com/watch?v=x8qB-PUw2AQ