Bottom cap rubber for trumpet / cornet, set of 3, "thin" type. *This is the "thin" type (photo shows "thick" type). When the sound becomes difficult to produce or you get tired, loosening the bottom cap can make it easier to play. This rubber is used to prevent the bottom cap from falling off even when adjusting its tightness, and it also improves the appearance. (Please note that valve oil may leak slightly when the bottom cap is loosened.) If anyone is interested, please let me know. Thank you. m(_ _)m