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A henley shirt is a collarless men's pullover shirt, characterized by a 10 centimetres (3.9 in) long placket beneath the round neckline, usually having 2-5 buttons. It essentially resembles a collarless polo shirt. The sleeves may be either short or long, and it can be made in almost any fabric, although cotton, cotton-polyester blends, and thermals are by far the most popular. Henley shirts are generally regarded as menswear, but recently women's versions have appeared as well.
They were so named because this particular style of shirt was the traditional uniform of rowers in the English town of Henley-on-Thames. Some crews still use this style as part of their uniform.
Originally quite popular in the early 1970s, henley shirts have recently made a fashion comeback, especially in Western countries such as the United Kingdom, United States, Canada and Australia, but also in some East Asian Countries such as Japan and Korea.
In his biography of Ralph Lauren, the journalist Michael Gross quotes a New York merchant who recalled showing a vintage shirt to a Ralph Lauren buyer: "I showed this fellow underwear---a three-button long-sleeve shirt by Johnstown Knitting Mills. He said, 'This is a new shirt.' That's where he got the idea for the Henley shirt." 
- Michael Gross, Genuine Authentic, The Real Life of Ralph Lauren, pg 22.