A French fly, also known as a waist stay, is a hidden extra closure with an inside button that is attached to an interior belt. It can be seen on both men's and women's pants, although more commonly on men's.
The French fly's function is to take the strain off the top of the zipper, allowing the flap of the fly to lay down smoothly and flatly in a more relaxed way. The French fly is most often found on high-end trousers and dress pants. The top closure on the pants can either be an outside button or a flat-hook closure.