Originally Posted by: mbs9So is this an either/or or a both? -- When playing/practicing, is it best/proper technique to use only the tip and "fingerprint" area of the thumb on the back of the guitar neck?

You'll get a definitive answer on this Christopher or Lisa from the pro perspective shortly I expect.

As a fellow novitiate, here's what I've found so far.

How many sizes do hands, fingers and fingerboards and necks come in? How strong are they. Age or injury, and possbly gender, will affect this.

The hypothetical ideal possibly works well for some with ideal fretting hand.

My observation from playing a lot with mine within the past twelve months is keep the ideal thumb positioning in mind as a guide to technique. i.e. to keep it as upright as possible rather than lapsing into the 'lazy banana' when fingering something like open F. I use sufficient pressure to achieve stability and the fretting pressure needed (I use its upper segment), but no more so it doesn't inhibit fluidity or induce undue fatigue.

Pragmatically, because hands & fingers vary in shape, size and strength, perfect placement is the guide, not the reality necessarily conducive nor relevant to every open or barre chord formation nor part scales lick improv.

In short you will move the thumb placement as required to accomodate this. Just be conscious of its positioning in forming whatever it is you are. Ultimately be guided by if the note or chord is sounding as it should, placement of the thumb facilitates the strength and stability required, comfortably, without inhibiting transitions.