Unlike pallets, pallet boxes can be stacked when they are fully loaded. The front end featured vertically stacked dual headlights. Further layers are then stacked on top. It uses an array of photosites that consist of three vertically stacked photodiodes. It was made of oak piles driven into the seabed with boulders stacked between. Such fuel pellets are then stacked and filled into the metallic tubes. They can be stacked up and glued together to make ornaments. Vertical farming is the practice of growing crops in vertically stacked layers. In other applications the trusses may be stacked vertically, and doubled as necessary. Typically the racks are stacked three high, with locker space per person. The holds are topped by hatch covers, onto which more containers can be stacked. Several columns are stacked together to add complexity. A larger disk may never be stacked on top of a smaller. The parts are then stacked in frames and stored. They could be stacked, or even built into walls.