Mattresses are typically sold as a set with a matching foundation, a mostly rigid support system that is designed to go directly below the mattress. There are a number of types of foundations, and it is always recommended to purchase a mattress and foundation together.
The main function of a mattress foundation is to fully support the mattress and absorb impact on the mattress. It helps to protect the mattress from unnecessary wear over its life.
The best mattress foundations are modular grids of square coils made with heavy-gauge metal, all in a wooden frame with supporting slats. Other types of mattress foundations are the all-wood posture box, the wood and metal semi-flex grid, high-density foam and a box spring.
Traditional box springs consist of many coils that provide support within a wooden frame, but sometimes these coils are not rigid enough. This can cause a mattresses to sag sooner than it would on a more rigid foundation.
The warranty on a mattress can be invalidated if it is not used on the proper mattress foundation. When buying a new mattress, check the terms of the warranty for foundation requirements.
Mattress foundations come in all the traditional sizes of mattresses. However, most crib mattresses are sold without a foundation, as crib frames have an adequate support system.