I had been practicing the minor and major 9th chords. I had some insights that I want to make sure make sense:
Is there any reason not to play an octave of the root in the left hand just to mentally “anchor” the chord?
Am I correct that the right hand in a major 9 chord is the minor 7 of the third of the root; and the right hand in a minor 9 chord is the dominant 7 of the third of the root? If this is so, if there any reason not to think of them that way?
I meant minor 9 chord Major 7 of the third of the root.
Hi Wendy,
Interesting questions here.
Doubling the root can add a lot of unnecessary ‘weight’ to our chord voicings.
Playing the root once is ample to establish the foundation of the harmony, and by doubling it we are creating redundancy in our voicing, ie. it is not adding any colour, or harmonic flavour to the harmony.
Whilst this may help you to visualise the chord in the short term, my opinion is that it would be better not to do this for your longer term development as a jazz musician.
When we come to explore more interesting voicing possibilities we can better allocate our fingers and thumbs by choosing other tones such as extensions and alterations.
Jovino created a relevant 5 Minute Masterclass which addresses your question. Here it is:
In particular he notes the importance of non-redundancy. See his example using a Bb Major chord at 0:47s.
Onto your next question:
Yes this approach can be used to visualise extended voicings. When starting out leaning major and minor 9th chords, by all means use this shortcut to help you find the voicing.
However, once you find the voicing, spend a second to make a conscious analysis of the notes you are playing in relation to the root note. Say to yourself “that’s the minor 3rd,” “that’s the minor 7th”, “that’s the 9th” etc… and that way you will be building your awareness of where these tones are located in relation to the underlying harmony.
The method you outline could be effective for building sequentially stacked chord voicings ie. 1-3-5-7-9, but that is just one way to build a chord. In practice, when playing jazz standards we don’t always stack the notes of the chord sequentially like that.
Instead we can rearrange, reorder, and omit certain notes to get different textures from our voicings and that is why it is vitally important to be able to visualise each scale degree of the chord in relation to the root note.
I hope this helps and any further questions let us know 
Cheers,
Hayden