About C#maj7b5 — uncommon voicings
C#maj7b5 is C#, E#, G and B# — 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 C#M7b5, C#Δ7b5 or C#maj7(b5).
What these less common shapes are
How to use C#maj7b5
How you get to it. The turn that lands on C#maj7b5 is to — the ii and the V of C# — and shares its third and seventh with , so the hand barely moves between them.
Hear it land:→→
In which keys. Every note of C#maj7b5 belongs to F minor (the ♭VI) and Ab major (the IV, the subdominant).
What to play over C#maj7b5
Every note of C#maj7b5 is in C# lydian and C# lydian augmented.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where C#maj7b5 sits in it. Every scale.
Chords near C#maj7b5
Change one note of C#maj7b5 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).
C#maj7b5 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.