When choosing a Schilke mouthpiece, there will be 1-4 characters in the size, each pertaining to a measurement:
1. Cup Diameter (First number in the size) - Diameter equates to .600" plus approx .005" times the size, give or take. Thus the diameter of a 6A4a is approximately .600" + .005 times 6, or .630". There are minor deviations from this but one can use this as a general rule.
2. Cup Volume/Depth (First letter in the size) - The Volume goes from A (shallow) to D (deep). Shallow gives you brighter sounds and helps you in hitting the higher registers. A deeper cup produces a darker sound with fuller tone. The C is a compromise cup size offering some of the qualities from both ends of the spectrum.
3. Rim Contour (Second number in the size) - The #4 rim is a little wider and flatter than the #3 (the #4 is similar to the Bach mouthpieces). The #2 is narrower, promoting flexibility.
4. Backbore (Second letter in the size) - C is the standard backbore, although Schilke also offers other sizes such as a, b, d, and e, with the 'a' being a tighter backbore and therefore more brilliant.
Please note: Where there are fewer than 4 characters in the size, one or more of the measurements is using the standard for that category and is therefore not listed. For example, a size 24 mouthpiece has the size 24 cup diameter and then the defaults for the other 3 measurements as follows: ‘C’ Volume, ‘3’ Rim Contour, ‘c’ Backbore.