The closer a mouthpiece comes to fitting your personal physiology, the greater the results you can expect to achieve. If you have thinner lips, youll probably want to go with the standard 17mm size. All Stork C series French Horn mouthpieces have a deep cup depth and an inner diameter of 17mm. They are available in 1, 4, 6, 8, 10, 12, 15 bore sizes.