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A bun (also known as a bobtail) is a type of hairstyle, typically worn by women, where the hair is pulled back from the face, twisted or plaited, and wrapped in a circular coil around itself, typically on the back of the head or neck. They can either be secured with a hairpiece, a hairnet and bobby pins. They may be tightly gathered or slightly messier and more informal. Buns, like ponytails, may cause headaches if worn too tight or up too high on the head.
In India, it's common for female college professors to wear their hair in a bun, in order to identify with their profession.
Men are not often seen wearing them, but they can do so. This may be because they are using it in a practical way such as in sport (like samurais) or using it in a formal way.
A bun worn high on the head is known as a "ballet bun." This hairstyle is used by nearly all female ballet dancers and some male ballet dancers, with a few notable exceptions. Ballet dancers often use hairnets and bobby pins to make the bun as tight and neat as possible. This is especially important while turning, as loose buns may fall out and the bun has to look presentable.
Double bun 
The term was popularized by the anime and manga series Sailor Moon, whose eponymous heroine, her mother during the Ancient Silver Millennium, Queen Serenity, and her future daughter from the 30th Century Silver Millennium, Chibiusa (although Chibiusa's are more conical than spherical), all wear their hair in this style. They also wear two pigtails of hair that hang down from the two spherical buns, but this is not universal to the term's usage within Japan, itself. The term odango in Japanese can refer to any variety of bun hairstyle.
In China, the hairstyle is known as "ox horns". It was a commonly used hairstyle up until the early 20th century, and can still be seen today when traditional attire is used. This hairstyle differs from the odango slightly in that it is gender neutral; Chinese paintings of children have frequently depicted girls as having matching ox horns, while boys have a single bun in the back.
See also 
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