A split box spring is a regular mattress foundation made up of two separate pieces placed side-by-side. Split box springs are made for delivery purposes. If your foundation is needed to be delivered to the place with tight doorways, staircases or narrow corners consider buying a split box. Most queen size box springs come in both whole and split types. All king size mattresses use split foundations. Only order split boxes if necessary since they cost more than a one-piece box.