Trying to get a handle on the theory of the 9th and 6th chords. Since the root note is not being played does that imply that for example the G6 chord is also the A9 chord and what you are playing depends on the context of the song?
The root is being play by the bass player or another instrument.
Justin gave the interval of the chord this and the sound is what matters.
b7, 9, 5 will become R, 3, 6 by moving to up a whole tone or step.
Lap steep guitar tune to the 6th chord, which is where I think this comes from .
There is also 3 inversion for the chord this first was the b 7th but you can also play the inversion with the 3rd or the 5th as the first note.
so you can moving these chords up a whole tone turn them into the major 6th chord.
the key thing to notice is that while playing the 9 no root which is an extended Dom 7th chord is the tone , it is kind of pushy restless more aggressive and the 6 chord is sweeter in sound.
yeah confusing cause it is the same shape but unless you are hearing unpaired you hear there is a great difference in the two chords.
it is base on the root and how it relates.
This is an advanced subject you will need to study more theory to get it but the shapes and the lesson can be used , I used this for many years cause it sounds good .
This is what Justin was intending all to do as well.