Many mistakenly call rims “embouchure visualizers,” but they have more uses than to observe the placement of one’s embouchure. The real value is to isolate the lip*s muscle fibers while buzzing. Jacobs cautions students not to buzz their lips without the use of a rim. Buzzing without a rim involves many more muscles than if the lips are supported with a rim. He recommends that buzzing should only be performed in the lower mid-range and for very short periods – two or three minutes maximum.