Before voices can be classified they must be brought out, and by the right method, so that their quality and category may be determined. Not every voice shows at the beginning to which category it belongs, and in such cases the teacher must proceed cautiously and not stubbornly to a preconceived judgment, from a wish to appear infallible. Unless the development is closely followed a fatal mistake is easily made.

Marchesi, Blanche. The singer’s catechism & creed. JM Dent & sons, limited, 1932.