The division of the singers into the epochs of their work is intended to make the list clear and usable. The division into smooth century boundaries was purely arbitrary - there are borderline cases with every division of time.
The assignment of the singers to the columns is roughly based on the period of the climax of the singing career. A subsequent activity as a professor, conductor etc. should not be taken into account for the classification. Singers whose careers are clearly on both sides of a century line can also be named in two columns.
Patrick Barbier: Historia dos Castrados (Portuguese version; title of the French original: Histoire des Castrats ), Lisbon 1991 (originally Editions Grasset & Fasquelle, Paris, 1989).
Michael Heinemann: Giovanni Pierluigi da Palestrina and his time , Laaber-Verlag, 1994.
Jürgen Kesting: The great singers. 3 volumes. Claassen, Düsseldorf 1987, ISBN 3-546-45387-5 . (One-volume, abridged and revised editions: The great singers of our century . Düsseldorf: Econ 1993; The great singers of the 20th century . Munich: Cormoran, 1998)
