Here is a video where I talk about this in the Rock Level 2 course.

Just a Second Lesson

To clear up some confusion, I am essentially referring to the same chord concept as "add9" but referring to it as an "add2." These are the same chords, and this is honestly quite a debate amongst guitarists in what is the right way to refer to this chord in particular.

"Add9" and "Add2" are the same chord. In the video I posted, those are also called "sus2" chords being that the 2nd, (or 9th), is being used in place of the third. If you "add2" or "add9", you can still have the major or minor third present and it sounds great. Once you have a 7th in the chord, than you will always refer to what was once a "second" a "ninth."

Make sense?