As with every part of the mouthpiece, it is never an easy concept to explain and there are multiple factors to look at. The rim shape is personal to the players mouth shape and playing techniques. Learn what rim shape works best for your face shape through trial and error. But keep in mind, a mouthpiece may feel great for 5 minutes but won’t work for you during an hour long recital. If you’re testing mouthpieces be sure to really put it through its paces.