(1) One 12" X 1/4" double-sided diamond grinding disc, #100 (medium/coarse 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.

Each side of the double sided disc is replaceable when it wears out or becomes too inefficient.