A pannier /ˈpæniər/ is a bag, box, or similar container, carried in pairs attached to the sides of a bicycle. There are many styles of bicycle panniers. Touring panniers come in both rear and front styles, usually sold in pairs, intended to hold enough equipment for self-sustained tours over days or weeks. Commuters who bicycle have pannier options designed to hold laptop computers, files and folders, changes of clothes or shoes and lunches. Since the movement against disposable shopping bags emerged, many panniers are made easily detachable from the bike, to allow using them for shopping bags. Bicycle panniers are usually made of nylon, leather or other synthetic fabric. As bicycles are often ridden in the rain, many panniers are built to be water-repellent or waterproof by themselves. Others include built-in rain-covers, or rain-covers are offered as accessories. The shape of the pannier may be enforced by a frame or stiffening panel made of plastic or metal to help keep it in place and prevent it from contacting a wheel. Panniers are usually built to attach to a rear rack or front rack already fitted to the bicycle. Removable panniers hook onto the top edge of the rack and are often held in place by a latch or elastic mechanism