[JonPR]

Originally Posted by Mtn Man

The important thing for you to worry about is your wrist angle. Try to keep your wrist nice and straight, and a little space between your palm and the guitar neck, and the thumb should naturally fall in the right location. The reason classical players have their thumb so far down on the neck is because they play with the guitar cocked up at a 45 degree angle. If the wrist is straight, this will naturally cause the thumb to sit lower on the neck. When you lower the guitar down to where the neck is parallel to the floor, it opens your elbow up, and if you keep your wrist straight, your thumb will naturally ride up the neck.

Well, yes, it can open your elbow up, but it's better if it doesn't.
I.e, when seated, transferring the guitar from classical left leg to rock(etc) right leg, the left wrist and arm angles should remain the same. The angle of the neck obviously lowers, becoming near horizontal. But there's no reason for the hand/wrist/arm position to change.

The more one's elbow opens out - you're right - the less comfortable it is to maintain classical thumb-behind position; it distorts the wrist too much, as you say. But the answer is not to change thumb position to keep the wrist comfortable; the answer is not to let the elbow open out.
The optimum elbow angle is 90 degrees (or less), and it's quite easy to maintain this with the guitar on the right leg; or indeed when standing with a strap.
A straighter arm - keeping a straight wrist, with thumb over or (worse) parallel with the neck - means the fingers are much more inihibited in their movement.

However, this is not necessarily a problem for playing simple rock stuff on a narrow-neck guitar - which is why such positions are so common!
But when playing acoustic seated, I think one should not let the elbow open out too much (pushing the neck forward or down); it puts the fingers at an inefficient, sloping angle. Again, it will work on simpler stuff, but is inhibiting for anything where the fingers need to stretch more. (And for beginners, of course, stretching is a common issue.)

Originally Posted by Mtn Man

Trying to play with a "classical" thumb position with the guitar in "folk" or "rock" or "bluegrass" position will put a lot of stress on your left hand and can cause fatigue or even injury. At the very least, it will inhibit your potential to play to the best of your ability.

Yes, "classical" thumb position with an opened-out elbow is bad. But that's because it's mixing two incompatible positions, nothing to do with the style of the music. (I know that's not what you're saying, just clarifying.)

What you call ""folk" or "rock" or "bluegrass" position" means thumb over (to maintain the straight wrist), and is fine for certain things. But proper classical left arm position - right angle elbow, thumb behind - is perfectly good in any of those genres (seated or standing), and is actually better when anything complex or stretchy is required.

Originally Posted by Mtn Man

I like to illustrate my points with videos of the best. Me talking about it is just words, but watch these guys. Don't feel like you have to watch each video in its entirety, I'm just illustrating my point:

Scott Fore, National Flatpicking Champion. Note how straight both of his wrists are for the entirety of this (very difficult) arrangement, even when he's fretting on the bass strings. Near perfect example of flawless right and left hand technique.

Bryan Sutton. Again, note the wrist, and how he allows his thumb to ride pretty high up the neck. This is due to the guitar neck being parallel to the floor.

What's interesting here, IMO, is that their left hand work doesn't require a lot of stretching. The complexity of what they're playing is mostly in the right hand, while the left rarely strays outside simple chord shape positions.
The thumb is mostly behind the neck - generally visible over the top, but rarely wrapping over.
Clearly the wrist and hand position is comfortable, relaxed, and does the job. But as I say, none of that is very demanding of the left hand (in terms of stretching at least).

Originally Posted by Mtn Man

Johann Pachelbel. Johann is a fairly high level classical guitarist.

Er, Pachelbel is the composer (died in 1706), and was not a guitarist! The guitarist there is Per-Olov Kindgren .

Originally Posted by Mtn Man

Note that the left hand looks completely different...but the wrist is still kept relatively straight. This is due almost entirely to the angle of the guitar neck. Because of this, and the wider neck, you don't even see his thumb. But the wrist is kept straight throughout.

Right. Note also the bigger stretches being demanded of the left hand. This is of course common in classical guitar, and is the main reason for the whole classical position orthodoxy: it's simply the most efficient, ergonomic way to put the whole neck at one's disposal and (given the wider flatter neck) allow the fingers maximum reach. (The only thing that position makes difficult, because of the right arm position, is rock-style strumming; but of course you don't get that in classical guitar; if chords are strummed, it's usually done rasgueado.)
The bigger the stretches required, the better it is to have the thumb on back of the neck (roughly central), and positioned in between index and pinky, or maybe opposite middle finger, so it supports all the fingers equally.
The tighter elbow angle also (along with the lower thumb position) makes it easier on the fingers, by allowing them to reach further across the neck.

But there are also positions between these two, which are quite common in various popular styles. I never play in strict classical position (even when playing classical pieces), but I often play with a left hand position quite similar to classical, at least if playing blues or folk fingerstyle, or rock or jazz barre chords with big stretches.

Originally Posted by Mtn Man

I just find that a lot of people focus way too much on the thumb, when thumb placement is merely a product of other (more important) things.

Very true!
I agree with you about the importance of a straight wrist, but I also think the elbow angle is critical. 90 degrees optimum, as I said. It can be less (as in classical) it can be a little more (as in most folk/rock positions). But the more it opens out, the worse it is for general fretting finger movement. (And beginners do have a tendency to straighten their elbows too much, as well as distort their wrists by laying the guitar flat so they can see what they're doing .)
I've found that if one thinks about elbow and wrist, the thumb position more or less takes care of itself. (It will be different in any case for open chords or barre chords, for scale/melody playing or for blues bending.)