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Overtone and undertone singing
In overtone singing, a singer holds one low note and shapes the mouth and tongue so that one harmonic becomes much louder than the others. It sounds like a second voice whistling a melody above the drone. The tradition is strong in Tuva and Mongolia, and among Xhosa women in South Africa. Some Tibetan monks chant with a very deep undertone: a second pair of folds in the throat vibrates together with the vocal folds and adds a note an octave below.