The opening of the mouth must be only sufficiently wide to admit the thumb between the teeth.
In singing the first exercise upon the vowel A, open the mouth, and fix its position, before emitting the tone, and not simultaneously. Keep the same position firmly, but without stiffness, throughout the entire vale of the note.
Should there be any difficulty in keeping the mouth steady, it will be useful to place a small piece of wood or a small cork between the upper and lower teeth, either on the right or left side of the mouth.
It is always interesting to figure out where we get our ideas and how the articulation of these ideas have changed (or not!) with time and research. To read the rest of the article, check out VoiceTalk here!