In classical singing, female and male voices are divided into voice types, also known as Fachs (German: Fächer). A singer's voice type may change during their studies or career. This research paper investigates the reasons for a singer moving to a different voice type as well as the results of the change, considering the singer from a psycho-physical point of view. The book introduces the reader to different voice type classification systems, including German and Italian systems, and discusses the central determining factors for the classification of a voice. There is also discussion of the importance of classification; when it is appropriate to classify a singer and when it is more a case of the voice maturing to its natural type, or the true discovery of the voice's correct classification, than an actual change of voice type.