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?

Thanks,

Jay