Originally Posted by: jwc10s

Okay, so I would like the math behind creating a add9 lets say for D. I understand the triad (1,3,5= D,F#,A) and to that you add the 9th so I believe that would be an E. So the Dadd9 chord would be D,F#,A,E. Now if I am correct so far, what I am missing is how to determine which note to change. In the charts it is changing the 3rd in the chord for the Dadd9. Is this standard if not what decides which note to replace?

You've got the right idea!

In any "add" (added) chord you don't change or replace any other note. You simply add that scale degree to the existing chord.

So, Dadd9 is:

D - 1st (root)

F# - maj 3rd

A - 5th

E - 9th

Any place you find on your instrument to play those notes together is a Dadd9.

In "sus" chords ("suspended") you "replace" the 3rd with either the 2nd or 4th. For example:

Dsus2:

D - 1st (root)

E - 2nd (replaces maj 3rd)

A - 5th

Dsus4:

D - 1st (root)

G - 4th (replaces maj 3rd)

A - 5th

Hope that helps!