Soft, molded material. Inside rails fit snugly against mouthpiece. Will not scratch your mouthpiece. Will not crack or chip if dropped. 
 
The D'Addario mouthpiece cap for Alto is made of flexible material and features rails that fit snugly against the mouthpiece. That way, if you accidentally drop your mouthpiece cap, it will not break or chip.
 
The D'Addario mouthpiece cap for Alto is made of flexible material and features rails that fit snugly against the mouthpiece. That way, if you accidentally drop your mouthpiece cap, it will not break or chip.
Makes less noise when dropped