If the pupils opened their mouths incorrectly, I would show them how to do it right; and by many practical demonstrations, I have finally fixed a general rule which I will give here…

This is: every pupil must shape his mouth for singing, just as he shapes it when he smiles. The upper teeth show a little, and are slightly separated from the lower ones. With many proofs and still more examples and much patience, I always obtained the desired result of discovering the necessary execution or such a rule. Besides, it is very easy, and in conformity with the best methods of the best schools. Let the teachers follow this method with their pupils and I dare to promise them the most satisfactory results. In giving them practical demonstrations, they can emphasize still more the truths of these rules by making their pupils pronounce the Italian vowels A-HA, E-A, I-E, O-OH, U-OO. With the above indicated position on the mouth, they will see that this position is not changed in going from one one vowel to another, except in pronouncing O and U.

—Giovanni Battista Mancini, Practical Reflections on the Figurative Art of Singing, Milan 1777, translated by Pietro Buzzi, Gorham Press, 1912: 93-94

Legion have been the singers who have attempted to integrate this teaching and have failed. Why? As indicated in my last post, the "smile" is applied from without rather than being an expression of the ear. Yet the truth is always before us: if it sounds right it will look right too. And this can be the hardest thing to learn. Why? More often that not, the student cannot look and listen at the same time; that is, he can't see what he looks like when he's singing well or correct it when he's not. As such, his auditory-vocal control is not yet functioning at a high level.

This lack of control is evident in the student who is instructed to stand in front of a mirror and observe or correct what he sees, and right before the sound comes out of his mouth, looks down at the top of the piano or to the floor. Why? The ear, which must focus just as much as the eye, is still learning to do its job.
It takes courage and skill to face your Voice.