This Mouthpiece Cap is a proprietary cap for Theo Wanne saxophone mouthpieces, designed and manufactured by Theo, to protect the mouthpiece with reed on. Theo Wanne mouthpieces come standard with the Reed Replacer Cap which protects the mouthpiece in storage and when traveling. Use the Reed Keeper cap when you wish to leave the reed on the mouthpiece, such as when you are on stage, or during practice sessions. The Reed Keeper Cap will STILL protect the mouthpiece tip, even when dropped. There is a size that for every current-version Theo Wanne mouthpiece.