Probably you have noticed that guitar players place their thumb on the guitar in many different ways. Some wrap the thumb around the neck and others just keep it at the back. The question is which way is correct?
I believe they are all to some degree correct. It comes down to the size of your fingers, the chords that you are playing (some chords require the use of the thumb to play certain notes) and the guitar techniques that you are using.
The most important thing to remember is, our thumb is there to support our fingers. They work together as a team. So different chords will require different thumb placements. Many times it won’t be a huge change, just small tweaks to make our fingers feel more comfortable.
For the majority of open chords, I would keep my thumb at the top of the neck (watch the video for the full explanation). But you may notice that it feels more comfortable when you wrap your thumb around the neck a little bit more especially when you have longer fingers. One point to consider is, if you squeeze the guitar neck too much, you might start to mute some of the strings and this is not a good idea when you play chords (although it might be the desired effect when playing solo techniques, as sometimes you need to play one note at a time).
The most important thing to remember is, our thumb is there to support our fingers.
When it comes to playing scales, I would position my thumb in a slightly different way. I would probably place it more towards the centre of the neck because it allows me to reach some stretchy notes. But if I would want to bend a note from the scale, I would reposition my thumb more toward the top of the neck. That way, my grip will be stronger and it will be easier to make bends and vibrato techniques.