“These two registers are used by every type of voice and they are easily employed when the correct technical mechanism of the vocal instrument is used. It is not necessary to sacrifice one register to the development of the other. Both are essential to the well-trained voice and both can be developed adequately. It is only when both of these registers have been produced correctly that great range results. With strict adherence to the proper manipulation of the vocal instrument for the production of these registers, separately and joined, it is the usual procedure, rather than the unusual feat, to develop a range of two and one-half octaves after even a short a time as six months of study. No singer lacking this range can truthfully call himself the technical master of his art.”
Hagara, Evelyn. Vocal Secrets of the Ancients. DeVorss & Co., 1940.