A Heavy Duty Racks warehouse is generally laid out with single-entry wall racks on the perimeter sides and double-entry racks in the middle. The working aisle measurement between each rack and the rack height depends upon the characteristics of the fork-lift trucks or lifting devices, pallet size and the height of the warehouse itself. Narrow aisle pallet racking is used for the storage of pallets with narrow aisles for specialized materials handling equipment to operate in.