These are the Ultra-Thin Straight diamond coated grinding wheels for use with the side grinder. They are designed for carbide blades with teeth that are very close together. The kit includes three 30mm and one 40mm wheels. This is different that the normal "Straight Thin" diamond grinding wheel...that version is best for carbide tipped blades that have more room between teeth and will likely last a lifetime. The Ultra-Thin kit has multiple wheels because, while they do last a long time, they are designed to be replaced periodically. I recommend getting both the Thin & Ultra-Thin wheels.