Adjustable shelving allows more flexible use of shelves to hold items of value for storage, display or sale. Like fixed shelves, the horizontal planes are normally made of strong materials such as wood or steel (occasionally glass or other materials), but their exact vertical positioning can be varied - usually through the use of uprights into which supporting brackets or the shelves themselves can be fixed at different heights. Purpose-built adjustable shelving systems can be fixed to a wall or free-standing.