Drive-in rack and Drive-through rack (sometimes spelled Drive-thru) are storage rack configurations that allow the forklift to drive directly into the lane of stacked rows (called a bay). The difference between a drive-in and a drive-thru pallet rack system is simply whether the bays have an entry at only one end, or at both ends. Drive-in rack systems use a common entry and exit, while drive-thru systems have entry points at either end of the bay Push-back pallet rack systems are designed around the principle of organizing space by depth rather than width. This depth arrangement greatly reduces aisle space and increases storage density. In this configuration, each bay can be up to six pallets deep; each pallet stored on wheeled carts that fit onto rails. The rails are slightly angled toward the load/unload side of the rack in order to take advantage of gravity, saving enormous amounts of energy for moving heavy pallets. Flow Racks are high density pallet storage systems that utilize depth to increase capacity. This system uses a slightly inclined rail with rollers that allow pallets to move easily along the sloped plane. These systems are also called gravity flow or dynamic flow systems. The pallet flow system often has complex motion and braking systems to control the speed of the moving pallet.