A box-spring (sometimes referred to as a bed base) is a hard sturdy wooden frame, covered in cloth, containing springs or some other form of torsion. Usually the box-spring is placed on top of a wooden or metal bedframe which sits on the floor and acts as a brace. The box-spring is usually the same size as the much softer mattress which is placed above the box-spring. Working together, the frame, box-spring, and mattress make up a bed. It is also common to find a box-spring and mattress being used together without the support of a frame underneath, the box spring being mounted directly on casters standing on the floor.

The purpose of the box-spring is threefold:

• to help raise the mattress's height, making it easier to get in and out of bed;
• to help absorb shock and reduce wear to the mattress; and,
• to help create a perfectly flat and firm structure for the mattress to lie upon.

In much of the world, beds do not need a box-spring, but most traditional western frame or four poster beds require one.^{[citation needed]} In Japan for example, a futon mattress might be placed directly on the floor or on a frame but never on a box spring.

The first rectangular spring-cushioned wire frames to support mattresses didn't have wood rims or cloth covers. These were called bedsprings. Currently more and more box-springs are being made out of wood, then covered in fabrics. Wood makes a better support system for the newer memory foam and latex mattresses. The newest design in box-springs is the folding box spring made of wood and springs, then covered in fabric which can fold in half and can be sent by shipping and courier companies.

With the increasing height of mattresses, manufacturers now make box springs in different heights. Standard "high profile" box springs are 9 inches (23 cm) in height, whereas "low profile" box springs are between 5 and 5.5 inches (13 and 14 cm). The difference between the two heights is purely aesthetic and makes no difference in the support provided for the mattress. However, the standard "high profile" box springs are considered to be more elegant in most areas and by many retailers.

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