I am of the opinion that vocal masters of the past were listening to the play of overtones in the voices which they were training – it is they who developed the term, “registers.” And, I know that better and faster results are obtained when a somnambulistic approach is taken in this acoustical type of singing rather than a rational, analytical, or intellectual one. To some singers vocalizing in this manner is their form of daily meditation. It sensitizes the thresholds of perception. Teaching by overtones leads more toward the art than the science of singing. It is the intuitive which inspires, not the mechanistic.

Coffin, Berton. Overtones of Bel Canto: Phonetic Basis of Artistic Singing: With 100 Chromatic Vowel-chart Exercises. Scarecrow, 1980.