The Pro-Line represents the darkest tone mouthpiece models we offer. This series features 3 rims (A, B and C) and 3 cup depths. The ‘A’ rim is our largest rim diameter most similar to a Bach 1. The ‘B’ rim is slightly smaller, most similar to a Bach 1 1/4 C. The ‘C’ rim is the smallest, most similar to a Bach 3C rim.