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A question I get pretty often is why one rim of the same inner diameter may feel larger or smaller than another marked to be of the same size.
Part of the issue is that mouthpiece makers are measuring a point on a curve, and one maker will likely have a slightly different concept of for example 18MM than another maker solely based on what point they are measuring to.
For years what I have found to be the more reliable way to tell if a rim has the same “feel” is to use a dime and directly compare how it sits on the rim when held upright, looking at where the letters of “United States of America” touch the rim. I learned this technique from a very handy publication, French Horn Mouthpieces: Material and Design published by Lawson Brass Instruments, where this illustration may be found. I have two versions of this pamphlet, one with a copyright date of 1990 and an older edition with no copyright date, from the late 1980s. Grip diameter is described in the section on “Selecting a Mouthpiece.”
The grip diameter can be determined if one uses a dime on edge to compare inside dimensions between mouthpieces. (See sketch).
When sighting along the edge of the rim, the position of the U and A, in the phrase “United States of America” can be compared between mouthpieces. The bottom edges of U and A just show in a .695” inside diameter rim.
This is an important dimension, as it is a good idea to reduce the number of variables you are changing when working with trying mouthpieces. If you have a rim you basically like, you would want to look for a new rim with essentially the same grip diameter and also what Lawson refers to as face shape, which is the relative roundness or flatness of the rim as it meets your face. You can and should experiment with different cups, inner diameters, and face shapes, but just not all at the same time, be systematic.
Give a dime a try! This technique really works. Adding to the above only to mention that the rim finish also impacts your perception of size, a slipperier rim will seem to be larger than one that stays more set on the embouchure.
