A well-constructed brass instrument mouthpiece should have a medium-wide rim with a fairly sharp inner edge. If the mouthpiece is properly placed, it will permit the lips to move slightly forward and backward. For high tones, a player will draw the lips farther back; while for low tones, the lip muscles will relax, permitting the lips to protrude. A sharp rim will not cut the lip if the flat face of the mouthpiece rim is placed on the lips in (or slightly above) a horizontal position, with the mouthpiece at a 90 degree angle against the front teeth. A sharp inner edge against the lip will automatically remind the player that the instrument is not being held correctly. The use of a mouthpiece without a sharp inner edge is not recommended, as it would not allow sufficient surface to distribute pressure over the lips. A too-rounded rim will dig into the lips, limiting the player’s endurance.
A player with a normal embouchure and fairly muscular lips should prefer a medium-wide rim, which will allow both flexibility and endurance. A toowide rim will clamp down lip muscles and embouchure flexibility, and the effect will be noticeable on quick tonal changes.
Players with very thick lips, however, can use a wide rim to advantage, as a medium-wide rim might dig into the soft tissues of the lips and interfere with the blood circulation. Players who cannot overcome the habit of “forcing” high tones, or band members who occasionally smack the mouthpiece against the lips while marching may also consider it advantageous to use wide-rimmed mouthpieces. However, even very thicklipped musicians and marching band musicians should prefer medium-wide rims if they do not feel hindered in using them, for mouthpieces with extra-wide rims encourage a player to use too much pressure for the high notes instead of relying on the lip muscles to do the work.
A narrow rim offers a trumpet ortrombone player greater flexibility, but it tends to dig into the flesh of the lips, cutting off free blood circulation and decreasing endurance. Horn players often prefer a medium-narrow rim because their instrument covers so wide a range (a fourth lower than a trombone and almost as high as a trumpet). The medium-narrow rim enables the horn player to move the lips much more easily; the lips will be able to protrude for the low tones and retract for the high tones.