I reckon a C major triad with a Bb major triad stacked on top is C11.

As I mentioned earlier, though, all those extensions could be flattened or sharpened. Each of them can then be written in a way that someone else could play them, but even then, the voicing can vary.

I once heard someone say "so we're adding extra noteds to the chord and that can only make it better, right?" Erm, well, I see his point, but I think I don't entirely agree :-)

But then one persons tension is another's dissonance.
We're balancing arguably boring against comfortable against surprising against shocking.

Jazz relative beginner, starting at a much older age than was helpful.
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