Flies

Flies are an opening at the front of trousers and men's briefs or boxer shorts to allow the penis to be taken out for toileting or other purposes without having to pull down the whole garment. Because of this, flies can be a fetish, especially for gay men. Flies on trousers are also often needed to allow the trousers to be put on, as most trousers are not stretchy enough to go over the hips without them.

Flies on trousers generally have some way to keep them closed, usually nowadays a zip though a few (especially jeans) stil have buttons. Underwear rarely has a fastener on the flies, although buttons are found occasionally.

Women's trousers usually also have flies to allow them to be put on. They are generally in the front, but as there is not the same toileting need as for men, it is possible to have women's flies at the side or rear of the waistband.