Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
Music: Practice & Theory Stack Exchange is a question and answer site for musicians, students, and enthusiasts. It only takes a minute to sign up.
I have never seen this kind of symbol before and google has failed me.
It's an 11-tuplet. Like a triplet, but with 11 notes where there would otherwise be 8 (or some other power of 2).
Required, but never shown