Originally Posted by: wallyflintI have a theory. I think that the REAL reason guitarists wrap their thumbs around the guitar neck so far that they are practically fretting notes on the low E string is so that their left arm doesn't get tired from having to support its own weight for long periods of time.[/quote]

It's more accurate to say that the role of the thumb is to provide stability.

The motions required by fretting hand & arm while playing guitar are far too active to let it be loosely hanging enough that it requires support. You don't want more tension than is necessary to fret notes. But that's always going to be enough to support its own weight!

Further, this would push the neck toward the floor which would result in poor playing posture & make it even harder to fret notes. I've seen players that use too much fretting arm effort to hold up the neck. I've never heard of the opposite, which is what it seems you are suggestion.

Originally Posted by: wallyflintI once took classical guitar lessons many years ago, so I tend to want to do the "correct" thing, which is putting my thumb against the back of the neck, directly opposing the fingers on the other side.[/quote]

This is a tricky area, because you see so many ways that various guitarists in different styles or even the same styles, place their thumb! And that's because different playing styles & techniques actually require different thumb placements.

So, any one-size-fits-all approach to "You must put your thumb here!" is bound to fail.

If you are playing classical style, then thumb behind the neck is best in most musical passages. That's because most classical music requires a lot of finger motion, dexterity & independence. Also most classical guitars have very wide necks.

Alternately, if you are playing blues licks with lots of bending, then thumb anchored on the bass side of the neck is best because you get the leverage required to bend the treble strings.

The right approach is to look for common general principles that hold true about the function of the fretting hand thumb. Regardless of style or technique. With all that in mind, I cover the fundamental mechanics of the opposable thumb in this lesson. One of the most important points that gets overlooked is the motion required by the thumb in order to provide support where it's needed at any given time.

https://www.guitartricks.com/lesson.php?input=12188&s_id=605

I show how to adjust those general principles to apply to blues bending here.

https://www.guitartricks.com/lesson.php?input=20406&s_id=1614

And how all this relates to classical style here.

https://www.guitartricks.com/lesson.php?input=10466

To be sure there are exceptions, like playing notes on the bass strings with the tip of the thumb. But those only serve to prove the point. Even when players do that they are still using most of the strength & dexterity of the thumb joint down to where it meets the wrist, while using the tip to fret notes! And they are using fretting hand fingers to provide additional support.

I've also seen rare times when players don't use their thumb for opposable pressure. But in this case they are fretting lightly, and using the pressure of the guitar body against the torso & picking arm in order to create stablity.

[quote=wallyflint]And my theory includes one more factor. When you support your left arm with your thumb, it makes your fingers more stable, as though they have a point of reference rather than having to just hit the frets in the right places through the thin blue air.

That's exactly what I mean by stability rather than support. The neck isn't supporting your loosely hanging thumb. Yout thumb is actively using the back of the neck as a point of stablity.

[quote=wallyflint]But the reason I struggle with this so hard is that when I hook my thumb around the neck, I feel it greatly restricts the motion of my fingers, especially the motion of spreading the fingers apart so that they are far away from each other.

This is going to vary a lot by individual player hand size & guitar neck width.

But in general you should aim to give you fingers enough freedom to fret the notes required while using your opposable thumb for stability & moving it as necessary. That might vary depending on the music you are playing.

Interesting thread, thanks! Hope my reply helps!