C#-alt7#9 Half-Diminished
The C#-alt7#9 chord is built from the notes C# – E – G – B – D##.
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Also known as
C#altadd#15C#m7#9b5C#m7b5#9
Chord tones, one by one
C#Root
Root
The fundamental note the chord is built on — C#.
EThird
Minor third
Three semitones — makes the chord minor (darker, sadder).
GFifth
Diminished fifth (♭5)
Six semitones — a flat fifth. Creates instability, typical of m7♭5 and diminished chords.
BSeventh
Minor seventh (♭7)
Ten semitones — the dominant/minor seventh. Adds bluesy tension (7, m7).
D##Extension
Sharp ninth (♯9)
A ♯9 — the gritty 'Hendrix' tension over a dominant chord.
🎼 Related scales
🎹 Modal analysis
This chord implies the Locrian scale; its characteristic note is the unstable ♭5.
🎸 Genres
Modern jazz · Post-bop · Fusion