Solid natural cork waterkey pad (spit valve cork). Tapered sides for easy fitment. Useable on most brass instruments from trumpet to tuba. Requires additional adhesive (not included) such as shellac or pad & cork cement to install. Size is 3/8-inch in diameter (about 9.5mm). Approximately 3/16-inch thick (4.8mm). Fits most brands of instruments. Can be filed down to fit others.