Definition
A fly front is a trouser opening where the zip is hidden under an overlapping flap of fabric at center front, as on jeans and tailored trousers.
A fly front has three parts. The zip; the fly facing (or fly extension), which forms the visible overlapping flap and is held by a curved, J-shaped line of topstitching; and the fly shield (or underlap), which sits behind the zip so it never touches the skin. A button or hook at the waistband closes the top.
Conventions differ: on women's trousers the fly traditionally laps right over left and on men's left over right, although many women's jeans now use the men's direction. The facing can be cut on, as an extension of the front piece, or cut as a separate piece, which is easier to keep flat.
Sewing a fly is a classic intermediate skill. It is more forgiving than its reputation suggests if you mark the topstitching line, baste the zip in place first and use a zipper foot.
Fly-front jeans are not in Sewly's library yet. The closest garments are the pull-on trousers and shorts, which have an elastic waist and no zip at all. For skirts and dresses, the usual closures are the invisible zip and the lapped zip, which is built on the same principle as a fly: a flap of fabric covering the teeth.