A bowler (also called a derby in the United States) was a type of hat with a domed top and a narrow, slightly-upturned brim.
In 1926, Man-Hat-On Milliners for Wizards ran an advertisement in the The New York Ghost noting the many hat styles they sold, including "derbys."
In 1991, the Sorting Hat sang a song to welcome students to Hogwarts School of Witchcraft and Wizardry, which included the line "You can keep your bowlers black."
Cornelius Fudge was noted to have worn a lime-green bowler during his tenure as Minister for Magic in the 1990s.
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