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Overtone singing – also known as overtone chanting, harmonic singing, or throat singing – is a type of singing in which the singer manipulates the resonances (or formants) created as air travels from the lungs, past the vocal folds, and out of the lips to produce a melody.
The harmonics (fundamental and overtones) of a sound wave made by the human voice can be selectively amplified by changing the shape of the resonant cavities of the mouth, larynx, and pharynx. This resonant tuning allows singers to create apparently more than one pitch at the same time (the fundamental and a selected overtone), while actually generating only a single fundamental frequency with their vocal folds.
Each note is like a rainbow of sound. When you shoot a light beam through a prism, you get a rainbow. You think of a rainbow of sounds when you sing one note. If you can use your throat as a prism, you can expose the rainbow – through positioning the throat in a certain physical way, which will reveal the harmonic series note by note.
It is thought that the art of overtone singing originated in southwestern Mongolia in today's Khovd Province and Govi Altai region. Nowadays, overtone singing is found throughout the country and Mongolia is often considered the most active center of overtone singing in the world. The most commonly practiced style, Khöömii (written in Cyrillic as Хөөмий), can be divided up into the following categories:
Mongolians also use many other singing styles such as "karkhiraa" (literally "growling") and "isgeree".
Many of these styles are also practiced in neighboring regions such as Tuva and Altai.
Tuvan overtone singing is practiced by the Tuva people of southern Siberia, Russia. The history of Tuvan overtone singing reaches far back in local history. There is a wide range of vocalizations, including Sygyt, Kargyraa (which also uses a second sound source), Khoomei, Chylandyk, Dumchuktaar, and Ezengileer. Most of these styles are closely related to the styles and variations in neighboring Mongolia.
Tuva’s neighbouring Russian regions, the Altai Republic to the west and Khakassia to the northwest, have developed forms of throat singing called "kai", or "khai". In Altai, this is used mostly for epic poetry performance, to the accompaniment of a topshur. Altai narrators ("kai-chi") perform in kargyraa, khöömei, and sygyt styles, which are similar to Tuvan. They also have their own style, a very high harmonics, emerging from kargyraa. Variations of kai are called karkyra, sybysky, homei, and sygyt. The first well-known kai-chi was Kalkin.
Tibetan Buddhist chanting is a subgenre of throat singing, mainly practiced by monks of Tibet, including Qinghai (Khokhonor) province in the Tibetan plateau area, Tibetan monks of Nepal, Bhutan, India, and various locations in the Himalayan region. Most often the chants hold to the lower pitches possible in throat singing. Various ceremonies and prayers call for throat singing in Tibetan Buddhism, often with more than one monk chanting at a time. There are different Tibetan throat singing styles, such as Gyuke (Tibetan: རྒྱུད་སྐད་, Wylie: rgyud skad) - this style uses the lowest pitch of voice; Dzoke (Tibetan: མཛོ་སྐད་, Wylie: mdzo skad), and Gyer (Tibetan: གྱེར་, Wylie: gyer).
The Ainu of Hokkaidō, Japan once practiced a type of throat singing called rekuhkara, which is now extinct. The last singer of rekuhkara died in 1976, but there are some recordings left. At sumo tournaments, the announcer, called Yobidashi, announces each wrestler's name using overtone throat singing.
On the island of Sardinia (Italy), especially in the subregion of Barbagia, one of the two different styles of polyphonic singing is marked by the use of a throaty voice. This kind of song is called a tenore. The other style, known as cuncordu, does not use throat singing. A tenore is practiced by groups of four male singers, each of whom has a distinct role; the 'oche or boche (pronounced /oke/ or /boke/, "voice") is the solo voice, while the mesu 'oche or mesu boche ("half voice"), contra ("against"), and bassu ("bass") – listed in descending pitch order – form a chorus (another meaning of tenore). Boche and mesu boche sing in a regular voice, whereas contra and bassu sing with a technique affecting the larynx. In 2005, Unesco classed the cantu a tenore as an intangible world heritage. Among the most well known groups who perform a tenore are Tenores di Bitti, Tenores de Orosei, Tenores di Oniferi, and Tenores di Neoneli.
The Sami people of the northern parts of Sweden, Norway, Finland, and the Kola Peninsula in Russia have a singing genre called yoik. While overtone techniques are not a defining feature of yoik, individuals sometimes utilize overtones in the production of yoik.
The Bashkirs of Bashkortostan, Russia have a style of overtone singing called özläü (sometimes spelled uzlyau; Bashkort Өзләү), which has nearly died out. In addition, Bashkorts also sing uzlyau while playing the kurai, a national instrument. This technique of vocalizing into a flute can also be found in folk music as far west as the Balkans and Hungary.
The resurgence of a once-dying Inuit tradition called katajjaq is currently under way in Canada. Inuit throat singing was a form of entertainment among Inuit women while the men were away on hunting trips. It was an activity that was primarily done by Inuit women, though men also did it. In the Inuit language Inuktitut, throat singing is called katajjaq, pirkusirtuk, or nipaquhiit, depending on the Canadian Arctic region. It was regarded more as a type of vocal or breathing game in the Inuit culture rather than a form of music. Inuit throat singing is generally done by two individuals but can involve four or more people together as well. In Inuit throat singing, two women would face each other either standing or crouching down while holding each other's arms. One would lead with short deep rhythmic sounds while the other would respond. The leader would repeat sounds with short gaps in between. The follower would fill in these gaps with her own rhythmic sounds. Sometimes both women would be doing a dance-like movement such as rocking from left to right while throat singing. The practice is compared more to a game or competition than to a musical style. In the game, Inuit women sit or stand face-to-face and create rhythmic patterns.
Some Thembu Xhosa women of South Africa have a low, rhythmic style of throat-singing, similar to the Tuvan Kargyraa style, that is called umngqokolo. It is often accompanied by call-and-response vocals and complicated poly-rhythms.
The 1920s Texan singer of cowboy songs, Arthur Miles, independently created a style of overtone singing, similar to sygyt, as a supplement to the normal yodelling of country western music. Blind Willie Johnson, also of Texas, is not a true overtone singer according to National Geographic, but his ability to shift from guttural grunting noises to a soft lullaby is suggestive of the tonal timbres of overtone singing.
Starting in the 1960s, some musicians in the West either have collaborated with traditional throat singers or ventured into the realm of throat singing and overtone singing, or both. Some made original musical contributions and helped this art rediscover its transcultural universality. As harmonics are universal to all physical sounds, the notion of authenticity is best understood in terms of musical quality. Musicians of note in this genre include Collegium Vocale Köln (who first began using this technique in 1968), Tran Quang Hai, Michael Vetter, David Hykes, Jill Purce, Jim Cole, Ry Cooder, Paul Pena (mixing the traditional Tuvan style with that of American Blues), Steve Sklar, and Kiva (specializing in jazz/ world beat genres and composing for overtone choirs). Others include composer Baird Hersey and his group Prana with Krishna Das (overtone singing and Hindu mantra), as well as Canadian songwriter Nathan Rogers, who has become an adept throat singer and teaches Tuvan throat singing in Winnipeg, Manitoba.
Paul Pena was featured in the documentary Genghis Blues, which tells the story of his pilgrimage to Tuva to compete in their annual throat singing competition. The film won the documentary award at the 1999 Sundance Film Festival, and was nominated for an Oscar in 2000.
Tuvan singer Sainkho Namtchylak has collaborated with free jazz musicians such as Evan Parker and Ned Rothenberg. Lester Bowie and Ornette Coleman have worked with the Tenores di Bitti, and Eleanor Hovda has written a piece using the Xhosa style of singing. DJs and performers of electronic music like The KLF have also merged their music with throat singing, overtone singing, or with the theory of harmonics behind it.
The Overtone Choir Spektrum from Prague, Czech Republic, is unique among overtone choirs, particularly because it connects traditional choir singing with overtone techniques. It is the only one of its kind in the Czech Republic, and one of only a few in the world. 
Several contemporary classical composers have incorporated overtone singing into their works. Karlheinz Stockhausen was one of the first, with Stimmung in 1968. Tran Quang Hai (b.1944), a French national of Vietnamese origin, created the composition "Ve Nguon" with the collaboration of Vietnamese composer Nguyen Van Tuong in 1975, in Paris. "Past Life Melodies" for SATB chorus by Australian composer Sarah Hopkins (b. 1958) also calls for this technique. In Water Passion after St. Matthew by Tan Dun, the soprano and bass soloists sing in a variety of techniques including overtone singing of the Mongolian style.