Dave Wirth
What's the worst piece of advice you can receive from a guitar teacher, ever? "You must have the thumb directly behind the neck."
Why? Thumb length varies from person to person. Advising someone to put it directly behind the neck basically ignores how their wrist looks and how long their fingers are. The worst part is that the thumb position affects the wrist position. If the wrist is messed up, then say hello to Carpal Tunnel Syndrome. Nasty, and probably not in a good way.
Carpal Tunnel Syndrome has everything to do with over-stressing the hand while it is in a horrible position. When the thumb is too high, then the wrist is cocked towards you. When the thumb is too low, the wrist is pushed away. Either way, a wrist that is bent unnaturally will have some problems later. The solution? Ignore everybody. Pay attention to the needs of your hand, and and place your thumb where it will keep your own wrist straight.
