C11 is C, G, Bb, D and F — with the eleventh added, and the third usually dropped.
It sounds suspended rather than tense. The eleventh sits a semitone above the major third, so the third is usually dropped; some shapes here keep it anyway.
Dominant — but effectively a sus chord with a ninth.
Also written C9sus4 or C dominant 11.
C11 has the notes of the ii over the V’s root, so it can take the ii’s bar: C11, then C9, then F. The eleventh gives way to the third, and the V resolves as usual.
Listen for: The eleventh falls a half step to the third, then rises a half step into the root of the I.
In which keys. Every note of C11 belongs to C minor, D minor, Eb major, F major, G minor and Bb major.
Every note of C11 is in C natural minor, C dorian and C mixolydian.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where C11 sits in it. Every scale.
Change one note of C11 and you have (drop the flat seventh), (the eleventh becomes the minor third), (add the minor third), (drop the ninth) or (the eleventh becomes the major third).
C11 is a dominant chord — root, major third, fifth, minor seventh. Dominant seventh chords on every root.
The common shapes and the derived voicings come from different places, and both are explained in how this site is built.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.