C#add9 is C#, E#, G# and D#. It adds the to without adding a 7th. That is what “add” means in the chord name.
The 9th has the same note name as the 2nd; the number does not require a particular guitar register. Compare with C#add9 to isolate it.
A repeated root rises a whole step to the ninth. The other notes stay the same.
That 9th can also tie two chords together. Play G# before C#add9: its 5th becomes the 9th of C#add9 without moving.
Also written C#(add9), C#add2 or C# add nine.
Going from Ab to C#add9, the ninth of the IV is the fifth of the I. Keep that note on top of both chords and it ties them together, which is much of why the pair sounds so good.
Listen for: The top note never moves: the fifth of the I, then the ninth of the IV. On the plain IV it drops a whole step.
Heard in“Overture” from Tommy
In which keys. Every note of C#add9 belongs to C# major, Eb minor, F minor, F# major, Ab major and Bb minor.
Contains C#add9 and adds sixth.
Contains C#add9 and adds fourth, sixth and major seventh.
Contains C#add9 and adds sharp eleventh, sixth and major seventh.
Adds fourth, sixth and flat seventh. The flat seventh pushes the sound toward a dominant chord.
Adds sharp eleventh, sixth and flat seventh, for a dominant sharp-eleventh color.
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.