In stores and online, mattress prices typically lists two numbers--one for the mattress and one for the mattress set. "Mattress set" refers to a base and a mattress sold together.


Some mattress sets include a mattress and a box spring, metal springs, or rods mounted on a piece of wood. Other mattresses come with a foundation, which is simply a solid or hollow block of wood.


With most mattress sets, manufacturers use the same fabric to cover both the mattress and box springs.


Mattress sets come in four main sizes--twin, or 38 by 75 inches; full, or 53 by 75 inches; queen, or 60 by 80 inches; and king, or 76 by 80 inches. Some people refer to twin beds as singles, and full beds as doubles.


The box spring or foundation provides much of the necessary support for the body during sleep, so buying a mattress set when your old mattress needs replacing is wise, according to the Better Sleep Council. Additionally, many stores offer discounts on mattress sets, or they provide a free frame or delivery when both pieces are purchased.


Platform beds and some toddler and day beds are built so that there is no need for a box spring or foundation. In such cases, it is beneficial to purchase only the mattress rather than the complete set.