Hi,
Thumb position is something that seems to tax quite few of us. I have a friend who was classically trained and he tried to insist that I should play with my thumb planted in the centre of the back of the neck, and not stuck up alongside where it usually was.
I showed him a series of pictures of every rock-n-rock great I could find, and
every single one of them had their thumb alongside the neck in the 'wrong' position like me. That shut him up for the time being.
I did try forcing myself to use the 'classical' position for a while, and I ended up with severe thumb pan that took a couple of weeks to mend. So I stopped that experiment. However, these techniques don't develop just to make life hell for students, they clearly do have a use in certain types of playing.
Currently, I've settled for comfort - and what I find actually works - over style. And what seems to work for me is to start with the guitar sitting on the web of my thumb (like most of us do). It gives me a lot of height and finger length to work with - so long as I'm working in a relatively close and comfortable area. Once I start playing it modifies a bit. The neck lifts off the web and the palm a little so that it's all a bit freer and I don't get the hand buzzing against the high E string. For certain types of playing, and certain angles and distances of stretch, my hand just seems to naturally change position and
slide round behind the neck into something close to the classical position. That feels fine too, but once the job is over it shows no inclination to stay there.
When you first start, the left hand tends to hold the neck in something of a 'death grip' but as you relax and improve then the thumb's job seems usually more about balance and contact than grip and pressure. Of course when you get to bar chords the thumb does need to slide round and apply pressure.
I'm always willing to be shown good reasons why other techniques are better. But until somebody can clearly explain to me why I should keep my thumb somewhere when it feels unnatural and uncomfortable then I'll stick with the way I do it now. If it's good enough for Kirk (check out his video lessons here) then it's sure as hell good enough for me.
Check this guy out too.
Naudo
He seems to know what he's doing. And you can see his thumb disappear round the back when he needs to use it that way, and then it pops back again. Looks good to me.
This is only the opinion of another learner though. If you can get a good explanation from a classical person, then do pay attention to why, how and when their methods might be better.
Cheers,
Chris