Hi Jericho2242,

Thumb placement is always a good question. The most accepted placement for your thumb when fretting or playing scales like spider exercises is to simply place it behind the neck. It does not necessarily have to be specifically between certain fingers, or anything. It's important to feel comfortable in your hand and wrist.

Remember, playing guitar is not science when it comes to your actual hand and finger nuances. You have to hit the right notes, that's the only rule.

I hope that helps.

Remember, relax!

Billy