You can choose your size option (S, TT, M, ML, MLL, L) after initial selection. Wedge two part backbores consist of an upper section, which can be regular or heavy weight, and a lower shank that can be of different lengths to adjust the gap, or for improving the pitch on a C trumpet. The two part systems provided great versatility in adjusting the backbore to your specific needs. The standard configuration is a regular or heavy weight upper section, as specified in your order, and a regular length shank that will produce the usual 1/8 inch gap in most receivers. Small - Slightly larger than Schilke A backbore. Produces a compact lead sound with good upper register support TT- Developed for endorsing artist Terry Townson. Intermediate in characteristics between Small and Medium backbore. Medium - Similar to Warburton 7 or GR #2 backbore. Medium-Large - Similar to Bach 10 backbore. This is an excellent all round backbore suitable for orchestral playing. Engstrom ML-L - This is the signature backbore developed for Howard Engstrom of the Calgary Symphony Orchestra. It is intermediate between the ML and L backbore. Large - This is our largest backbore, similar to a Bach 24. This produces a broad orchestral sound when paired with the appropriate top.