Tonsure means to cut off someone’s hair. Monks and nuns in the Middle Ages generally showed that they were monks by cutting their hair short (when most people wore theirs long). Christian monks got a special short haircut by shaving a bald spot in the middle of the back of their heads.
Among the Franks, just having short hair by itself seems to have been enough to keep you from becoming king. And among the Visigoths, both men and women had their hair cut off as punishment for a wide variety of crimes. And again, tonsure would keep men from becoming kings. It’s possible that this idea that cutting your hair is bad comes from the Greeks and Romans. They cut the hair of people who were enslaved, to make them look different from free people.
Buddhist monks and nuns in India and China and Central Asia didn’t get tonsures, but they often shaved their heads completely. That’s pretty much the same idea, that they were setting themselves apart from regular everyday people.
Learn by doing: what do we learn about people today from their hairstyles?
More about Christian monks
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Or more about medieval clothing