The registers of the singing voice, as commonly understood by singers, refer to the different vocal qualities induced by adjustments at the level of the larynx and of the vocal tract. This explains why register unification can be approached either one or a combination of the following procedures: (1) resonance alignment through vowel modification, (2) register alignment through intensity exercises. The wide-spread acceptance of vowel modification has made singers reluctant in exploring other avenues of register development. If registers are laryngeally derived, there should be another way of register unification, which directly addresses the coordination of the laryngeal muscles. In support of this argument, this thesis investigates the teaching practices of a group of twentieth-century American voice teachers, who rely on intensity manipulation as the primary means for enhancing the register adjustments. Intensity exercises such as the messa di voce has long been practiced in historical pedagogy, but it is not until now that voice science confirmed its significance in register coordination.