(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.

You get 3 grinding surfaces, two 12" in diameter and one10" in diameter.  The 10" very coarse disc can be easily popped on and off the main 12" double-sided disc as needed, and automatically attach by magnetic force. 

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