Sound adjustment nut
The nut adjusts tension on the cord. By tightening less, the sound gets darker and a reed vibrates more. It also compensates thickness of a reed. If a reed is too thin, tightening more makes the reed sound like a thicker reed - and vise versa.
Come with OmniCap that is small enough to fit in a narrow case, secures and protects delicate mouthpiece tip and reed, protects mouthpiece rail, preserves reed moisture, is easily washable and makes no sound when dropped.