In bluegrass singing the harmony part pitched at an interval above the melody is commonly called singing "tenor" even though it might be sung by a soprano. The term might have slipped over from the traditions of barbershop singing, I guess, where it was usual for a man with a high-pitched voice to sing that part. Or perhaps it comes from white gospel which is often similarly arranged. In any case it describes a function of singing a higher harmony, not the constraints of the voice singing it. At least that's the usuage as I understand it.

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