There is no fixed rule that can be laid down in regard to the necessary opening of the mouth and its position. The structure of the palate and the form of the jaw, and the position of the teeth, lips, &c., vary in different persons. The ear of the teacher must alone determine what position of those several parts will best secure a good timbre. But in every case, for the highest tones of the voice the widest possible opening of the mouth is necessary, and even when, in the formation of the vowels, the lips have to be brought nearer to each other, yet the teeth within must be kept apart, that the cavity of the mouth may remain large enough. Wind instruments show the influence which the orifice and breadth of the bell has upon the strength of the tone. In the human voice the mouth occupies the place of the bell. We have already made the remark, in speaking of the different registers, that in the chest tones the position of the larynx is lowered. The cavity of the mouth, then, is naturally lengthened, and hence a moderate opening of the mouth, so that, in singing the notes of the low chest register, the teeth are a thumb's breadth apart, suffices for a good tone. The second chest register requires the slightest opening of the mouth. It is enough if one can press a finger between the teeth. With the high falsetto and head tones the cavity of the mouth is always shorter and narrower towards the back, but as the tones ascend, it must be always broader in front. In singing the first falsetto register, the teeth should be about the breadth of the thumb apart; in the second falsetto register, two fingers apart; and in the head register, the mouth must be open as far as possible. But precise rules cannot here be given. I have observed, however, that in thin voices a too broad opening of the mouth in the middle tones of the voice favors the high over-tones more than the fundamental tone, and the tones are thus flat and wanting in timbre. Lips too thick and stiff sometimes injure the timbre of the tone; they are often the cause of a veiled, muffled timbre, acting like dampers and rendering a part of the over-tones inaudible. In such cases, as soon as he has become accustomed to a correct direction of the column of tone, the pupil should keep the lips as close to the teeth as possible, and draw back somewhat the corners of the mouth.