Silly question - left thumb position.
I learned all the open chords on an acoustic guitar when I was 10 from my primary school teacher, and didn't really take things much further than that, except a couple of e-shaped barres. I can strum along and figure out chord sequences to most songs, this was all I wanted to do at the time. Now I'm 22 and I've got myself an electric, and I'm starting to learn lead.
When I play scales and solos, I try to keep my thumb in the middle of the neck, so I can stretch, but when I'm playing open chords I'm in the habit of having it up over the fingerboard and the neck nested between my thumb and forefinger. I'm finding it a really hard habit to break and I'm trying to keep my thumb in the middle of the neck, but chord changes are harder that way. Does it matter? I've started to take guitar far more seriously and I want rid of all the things I shouldn't be doing sooner rather than later. I've seen photos of guitarists on stage and they have their thumbs sticking way out sometimes, but which is the best way to be doing it?
The best way is the way that works best for you.
Nobody cares how much you know,
until they know how much you care.
"What you keep to yourself you lose, what you give away you keep forever." -Axel Munthe
Welcome aboard. As Tim suggested, there aren't any set rules; use the position that allows you to play
with the least amount of muscular tension.
I, persoanlly find myself switching between the two position quite often.
For example, I use the rock position (thumb over the back) for most of the open chords.
However, I use the classical position ( thumb in the middle) for barres to get the required stretch.
I also use the classical while playing two notes on the same string separated by 4 or more half steps (again, for the stretch).
In lead playing, especially involving bends and vibratos etc, you will need to use the rock position to get the required force, stability and control on the string.
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As long as you're not squeezing with your thumb, pressing the neck into your palm, or putting your thumb in places that create funny wrist angles you're fine. The only reason to put your thumb behind the neck is to increase your reach. That's a good reason, but you don't need to do it all the time.
thumb over the neck for open chords is fine.
in fact I often use the thumb to press the low E string to sound it.
in this way I am using the F position chord w/thhumb over the neck hitting the first fret to give some bottom to the F chord.
keeping that same position move up two frets and you now are playing a nice G chord.
same position upo the neck and you have all the other major chords.
this is somewhat a looser version of full on barre chord.