Bottom cap booster rings for trumpet. Set of 3, copper, used. These rings are inserted into the bottom caps of trumpet valves. Simply opening the bottom cap and inserting a ring before tightening creates a moderate resistance, which is said to improve the tone. Some people use all three rings, while others use only one in a specific valve. Since results vary depending on the instrument and the individual, experiment with different combinations to find the number and placement that suits you best. (I always put the brass (medium) ring in the 3rd valve.) These are used items and therefore slightly dirty. That's why I'm offering them at a low price. If anyone is interested, please let me know. Thank you. m(_ _)m