KELLY Small-Shank Trombone and Baritone Mouthpieces are made entirely of Lexan® material. The 6-1/2AL and 12C cup sizes are patterned after the Bach® series. All cup sizes are sampled by our professional staff and are adjusted to produce our final, comfortable, easy-blowing mouthpieces. All of our mouthpieces are identical to exacting standards. FEEL the Comfort!