Final answer:
Countertenors commonly sing in the genres of classical, opera, and early music like Baroque. Their voice is characterized by the ability to sing in a high 'falsetto' range that's typically reserved for female alto singers. They became particularly popular during the Baroque period.
Step-by-step explanation:
Countertenors commonly sing in the genres of classical, opera, and sometimes in early music like Baroque. The element that identifies a countertenor is the ability to use the head voice to sing notes typically sung by female alto singers. Varying from male alto, which is a voice category based on vocal range, countertenors usually sing in a higher range called 'falsetto'. Famous examples of countertenors include musicians like Russell Oberlin and Philippe Jaroussky.
The countertenor voice became more popular during the Baroque period when composers began writing more music for it. Some modern-day ensembles, particularly in the realm of early music, still use countertenors as integral parts of their vocal structures.
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