I've been useing a Cavelier Elite Cordoven tab for close to 2 years now, was a bit strange to get used to, but once used to it, it works great. It does have a shelf, or thumb piece that you can use if you choose to, it attaches seperatly with 2 ellen screws. I use mine, my thumb goes right over the top and it insures that my thumb and hand configuration is the same on every shot. It also has a metal plate on the face where the Cordoven leather attaches to, and it insures the exact string placement on each shot. It really helped my accuracy. I have several now, it takes awhile to break them in and I tend to loose things!
I am sold on them. DK.