Watch the Add9 and Add11 Chords online guitar lesson by Matthieu Brandt from Chord Cookbook
When we leave the major or minor third in the chord and add the sus2 and sus4 notes, we end up with chords that have a different sound and name. Adding the sus 4 and leaving the major third gets you an add11 chord. Leaving the minor third and doing the same creates a minor add11. Adding the second note up from the root will create an add9 or minor add9. These numbers refer to the added notes one octave up.