Well, as you know they're are many many chords out their, and they're all found in different way's, this is the the first in a series of lessons to explain to you the different types of chords and how to relate to playing.
Now you remeber the degrees of the major scale...
Now 8 is the octave right what if we decided to keep going till the next octave?
Now when it comes to major harmonization, you use every other note, this how we originally got major chords, by taking the every other note till we had three notes, the 1, 3, and 5 notes. If we kept going we'd also have a 7 note, this is how you get a Maj7 chord, a typical 7th chord is actually a dominant 7th chord, meaning it has a flatted 7th, I dont know why. But if we take one more step, we have the 9, a 9 note is a peculiar things cause it's beyond the octave, to make it simple, it's a 2 that's an octave higher. So what notes does a 9th chord have?
Just a note of interest, the arpeggio for this chord you'd think would be a penatonic scale, but if you think about it, it's not cause it has no 2nd to compensate for the 9, so the order you'd play notes in key of C would be...
C E G B D
Trying to create a pattern that adheres with this would be a nightmare, so instead change the 9 to 2nd and you could use that as a penatonic scale over the chord.
Now you've gotten some pretty neat information but where do Add Chords come in, now playing 5 different notes in a chord is a difficult thing to do, and find convienently on the fretboard, so people usally decide to remove the 7th notes and just play 1-3-5-9. Now since this is not the next harmonic movement, you are just adding the 9, thus you get a add9 chord, more specifically in this case, a major add9 chord. This also is the case with add11(4ths Octave) and add13(6ths octave) chords. Now add chord due to the harmonic structure usally have nice harmonic sound, unless your playing an minor add chord or some other add chord variation. Well here's a Cadd9 chord to get you started on the way of the add chord.