Many “Catters” have posted a lot of tips and techniques on placement of the thumb and ways in which it can be used. Try reading the PermaThreads™, you'll find a lot of the discussion that way. The bottom line is, learn to take advantage of all useful thumb positions.

Rick has posted his thoughts on thumb placement in quite a few threads. Some of them are referenced—in no particular order—below.

Our Friend the movable B7 chord.
RE: ritchie havens chords
RE: Ouch!
RE: Questions about G, D, E7, and A7
RE: BS: I've Got A Sharp Axe To Grind
RE: MUDCAT VOW.
RE: MUDCAT VOW.
RE: Help: Learning to finger pick
Eeeee! Flat breads are delicious!
Does Mudcat Seem to Be flat Right Now?
What the 'F' is goin' on at Mudcat?
RE: chord inversions. rick needs help!

      - Mark