This song is all about math - and psychedelic, hippie nonsense. And odd musical time signatures. Common time in music is sometimes also called 4/4 - that’s 4 beats in 1 measure. The Eleven is an odd time signature known as 11/12. They drop the last beat. So instead of the beat going 1234, 1234, 1234 it goes like this: 123, 456, 789, 10 11. That drives the beat really fast and the Dead would morph common time into The Eleven and back again over the course of, usually, about 10 minutes. Lots of fun.
Niiiice! I'm pretty sure Mickey Hart was one of the early rock drummers to experiment with polyrhythms etc, and was influenced by the Nigerian percussionist, Babatunde Olatunji, who came to America in the late 50's. Thanks for the great explanation of 11/12 time, although I can't always find the downbeats myself as I'm listening. Tricky stuff! The Eleven: https://m.youtube.com/watch?v=J5McdXcZKbw