“Keep the thumb back” is something that many guitar students have heard over and over again. It’s not just keeping the thumb back that’s important, but rather using the thumb position to accommodate finger placement. When learning how to read music you can keep your thumb in one place behind the neck so you’re fingers can stay in position over a certain area of the frets. But when it comes to playing chords, you have to give the thumb a little more latitude and not be restrictive so it can accommodate the best finger placement on the strings and frets. Some players even use the thumb for lower notes of barred chords bringing the thumb completely over the neck, which I don’t recommend or do myself. And there are always those students who say, “Well, Jimi Hendrix used his thumb to play chords,” and that’s true, but you have to consider the consequences. Jimi Hendrix is dead.