Where does my left thumb go when I play guitar you ask?? Well it depends on what you're playing, but here are 2 positions your thumb should be in:
1) Pressed into the back of the neck
2) Guitar thumb muting strings
For intense scales, bar chords and shredding, you're going to want your left thumb in the back on the guitars neck. This stabilizes your hand and because of your hand placement (yer thumb!!) it makes your fingers longer! Yup. Stubby finger problem solved!
For your open chords and blues soloing, it's suuuper handy having your thumb hanging out the side and allowing your entire palm to embrace the neck of the guitar. Mmmm sweet embrace. But really! The reason for this is it can mute unnecessary strings from ringing
Try touching your loose thumb agains the big E string when you play a C Major chord - never have it ring accidentally again!
I hope this helped you with your left thumb placement on the neck of your guitar! Any questions or if you want to get together for a 1 on 1 guitar lesson, and if you want free guitar lesson videos, head to my website.