C#7#9 is C#, E#, G#, B and E — with the ninth raised a semitone.
It sounds gritty and contradictory. The minor third sounding over the major third; every guitarist knows it as the Hendrix chord.
Altered dominant — dominant function with a blues third on top.
Also written C#7(#9), C#7+9 or C#7♯9.
C#7#9 can replace a plain seventh chord wherever a bluesy sound is wanted, even as the first chord of a tune, where other altered dominants don’t belong. It sounds like home, with no urge to fall a fifth.
Listen for: The sharp nine, a minor third sounding over the major third, holds through the IV7 as its flat seventh.
Like the other altered dominants, C#7#9 works in front of a chord a fourth higher, such as F#maj7, a chord a half step lower, or a minor chord on its own root.
Every note of C#7#9 is in C# half-whole diminished.
Each of those is a page: the scale across the neck, and the chords it harmonises into — which is where C#7#9 sits in it. Every scale.
Change one note of C#7#9 and you have (drop the major third), (the major third becomes the ninth), (drop the sharp ninth), (the fifth becomes the flat thirteenth), (the sharp ninth becomes the ninth) or (the sharp ninth becomes the flat ninth).
C#7#9 is an altered chord — a dominant seventh with its fifth or ninth moved. Altered dominant 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.