Hello, I have a book about guitars for beginners, and in there it says that my thumb has to always be straight and vertical. I don't know, but when I'm playing fast across the fretboard and i need to stretch my hand alot, my thumb just auto shifts from this position | to -- ( horizontal , sometimes a bit diagonal ). For me, it's just more comfortable like this. So do i really have to always have my thumb straight, vertical?
It really come down to whatever's more comfortable. If you're playing chords and stuff where you're fingers aren't stretched across the fretboard, then try and keep your thumb up and down, about the same spot as your middle finger is. Other than that, have at 'er.
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keeping your thumb vertical helps balance out the pressure your fingers exert towards the fretboard, not doing so leaves your pinky and ring finger less controlled because of the offset of balance.

and in my opinion, its also less comfortable to have your thumb horizontal.
it should be vertical or nearly so at all times.shift your thumb as u move your hand during a stretch and its much more comfortable

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