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A turtleneck is a kind of sweater that has a small round piece covering your neck. Depending where you live, it can be called a polo neck, roll-neck, or skivvy. It can be thin or thick, worn tight or loose. There is a similar kind of sweater called the mock neck.
Turtlenecks have been popular in the west parts of Europe for a long time. They have been trendy in North America since the mid 1900s. People of any gender can wear a turtleneck. People wear them when it is cold.