Fly (clothing)

A fly (UK: flies) (short for flyers) is a strip of material covering an opening on the crotch area of trousers, closed often by a zipper, or by buttons. It may also appear on certain men's undergarments such as boxers or briefs to allow for easier urination, known as a keyhole fly. On men's garments, the fly always opens on the wearer's right side; on women's garments, it may open either on the left or on the right.^[1]

A fly can also be on other garments, like the paletot coat of the 20th century, where it is the front opening that can be secured close and is covered hidden by fabric.