Warburton mouthpieces are favored by studio and big band musicians. Compatible with Kanstul, Parke, and others. Throat size is #28 (3.57mm). This is a tight-type backbore, Backbore 2. Warburton backbores come in 12 sizes, from 1 to 12, with 7 as the standard. This design offers moderate resistance and is particularly effective for stable, brilliant high notes. Please feel free to ask if you have any questions.