As the title asks: What is the significance of this particular haircut (the tonsure), why is it associated with monks and what is the history behind it?
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[...] MONASTIC STYLES
Both men and women traditionally had their hair cut or removed in specific ways when they entered a monastery or convent. These haircuts symbolized religious devotion, group identity, and humility as well as the renunciation of worldly things and personal vanity. The practice may relate to ancient rites in which people in various cultures offered their hair as a religious sacrifice. Monks and nuns also take a vow of celibacy, and hair has historically been associated with eroticism and sexuality and as a means to attract the opposite sex.
Historians say that monastic hairstyles also may relate to the ancient custom of shaving the heads of male slaves. Some early monks who began shaving their heads voluntarily referred to themselves as "slaves of Christ." Such hairstyles thus would show that a person entering religious life intends to subordinate his own will to the will of God.
Groups of Christian men began to form organized religious communities during the second and third centuries. These men, who became known as monk, lived apart from other people and developed distinctive modes of dress and appearance. Some monks cut their hair short, while others shaved it off completely or shaved part of their head.
Partial shaving may have its origins in ancient Egypt, Greece, and other places where men shaved a circular bald spot on top of their heads to honor the sun god. Some orders of monks who left a narrow crown of hair around their heads said that this signified the crown of thorns placed on Christ's head during his crucifixion.
The distinctive style, which is called the tonsure (from the Latin word tondere—"to shear"), often is associated with Catholic monks. Historians are unsure about the earliest origins of the tonsure, but church officials came to accept it and then required that all Catholic monks adopt this hairstyle. The tonsure is "a sacred rite . . . by which a baptized and confirmed Christian is received into the clerical order by a shearing of his hair" (The Catholic Encyclopedia).
In Roman Catholic monasteries, novices who had just entered the community had their hair cut short with scissors. When the novice took his vows to become a monk, he received the tonsure. The hair was cut short and then the hair on top of the head was shaved off, leaving a round bald area on the crown. These haircuts were carried out as part of the initiation into the group and were maintained by monastic barbers.
Three main variations of the tonsure developed among various orders. The eastern style involved shaving the head completely, according to a style attributed to St. Paul, while others shaved just the crown, a style associated with St. Peter and known as the Roman tonsure. A third style, called the Celtic (or transverse tonsure or tonsure of St. John) evolved in the British Isles. Celtic monks shaved the front part of their head from ear to ear but left the hair in the back hanging longer. Some Celtic monks pulled that hair around to form a semi-circle from one ear to the other. [...]
Sherrow, Victoria. Encyclopedia of Hair: A Cultural History. Connecticut: Greenwood Publishing Group, Inc., 2006. Print.