Designed and engineered by the U.S. Postal Service, these Rack Ladder P.O. boxes can be installed in a wall between Rack Ladders or in a Rotary Enclosure. Five mailbox units per column (requires two rack ladders). Each additional column in the same configuration requires an additional rack ladder. This customizable 2-door P.O. mailbox system is great for use in universities, postal centers, hospitals, mailrooms, and apartment complexes.