I've been playing for a little over four years, and I have been taking lessons since I started. Now maybe its because they don't know how to play, maybe its actually a way of playing, but I noticed that alot of kids (not really adults) place their thumb behind the neck and wrap it over the E string. Is that right? It sure looks wrong to me. I was taught to keep the thumb stiff, and against the neck. But then I have heard some bassist actually play some notes using their thumb. But I think that is kinda wrong. Feel free to correct me if you want to.