Trumpet Bottom Cap Booster Rings Brass (Medium) Set of 3, Used. These are used by opening the bottom caps of your trumpet valves and inserting the rings inside before tightening, which creates a moderate resistance and improves the tone. Some people use all three rings, while others only use one in a specific valve. Since results vary depending on the instrument and 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 only.) 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