(1) One 12" X 1/4" double-sided diamond grinding disc, #120 (medium grit) on one side, #400 (fine) on the other side, two discs in one.  This disc fits directly on the potter's wheel just like a bat, and you just flip it over to change grits.

(2) One 10" #80 (very coarse) electroplated diamond disc with a magnetic backing.

(3) One 10" #800 (very fine) electroplated diamond disc with a magnetic backing.

You get 4 grinding surfaces, two 12" in diameter and two 10" in diameter.  The 10" discs can be easily popped on and off the main 12" double-sided disc as needed, and automatically attach by magnetic force.  This gives you more options, extend the life of the main disc and save you time.