About Bbmaj7b5 — uncommon voicings
Bbmaj7b5 is Bb, D, Fb and A — with the fifth lowered a semitone.
It sounds hollow and hard to place. Far more often met spelled the other way round, as a ♯11 over a chord that kept its fifth.
Coloured tonic — rare, and unstable for a major chord.
Also written BbM7b5, BbΔ7b5 or Bbmaj7(b5).
What these less common shapes are
How to use Bbmaj7b5
How you get to it. The turn that lands on Bbmaj7b5 is to — the ii and the V of Bb — and shares its third and seventh with , so the hand barely moves between them.
Hear it land:→→
In which keys. Every note of Bbmaj7b5 belongs to D minor (the ♭VI) and F major (the IV, the subdominant).
What to play over Bbmaj7b5
Every note of Bbmaj7b5 is in Bb lydian and Bb lydian augmented.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where Bbmaj7b5 sits in it. Every scale.
Chords near Bbmaj7b5
Change one note of Bbmaj7b5 and you have (the flat fifth becomes the fifth), (the flat fifth becomes the sharp fifth), (the major third becomes the minor third) or (the major seventh becomes the flat seventh).
Bbmaj7b5 is a major chord — root, major third, fifth. Major 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.