I have been playing guitar for a year and a half self tought and always kept my thumb over the neck of guitar. But now watching youtube videos i saw that all of the other guys keep it behind the neck and can do insane stretches like 3-7. Is my technique wrong?
Depends on what you play. If you're bending, put the thumb over and **** the wrist.
If you're playing something else that doesn't need the thumb, general rule is to keep the wrist straight and relaxed.
Find a good position for your thumb so your wrist is straight.
I tend to move between the two depending if i'm playing rhythm or lead.
3-7 isn't that much of s stretch really, i have relatively big hands but find 3-8 comfortable. If you're finding you can't make a 4 fret stretch then you're hand position is limiting you're ability.

try playing a song lik this to improve your stretch:
The only 6 words that can make you a better guitarist:

Learn theory
Practice better
Practice more
The only times the thumb goes over the neck are for bends and vibrato. The rest of the time it should be behind the neck.
Speed is a by-product of shut the fuck up.
i also think the size of your hand should be taken into consideration aswell...i have some really huge hands lol, fingers and thumbs are long so my thumb tends to stay partially at the top with about an inch pointed up past the 6th string and sometimes in the center for anything focused on the first 3 strings , but my wrist is always straight and relaxed, the only time ironically its not relaxed is when i force my thumb to stay in the center 100%...been playing this way for 18 years soooo...cant teach an olde thumb new tricks i guess lol.
Last edited by WaltTheWerewolf at May 20, 2010,
I always keep my thumb behind and parallel with the neck
"I'm not retreating, I'm initiating an offensive manouvre in the opposite direction!"