It is possible to place a mattress directly on the floor but more commonly they are placed atop a box-spring. Three items often make up a bed - a frame, a box-spring and a mattress. The frame is usually metal or wooden and it is essentially the stand within which the box spring fits. The mattress is then placed atop the box-spring. For obvious reasons the box spring is usually the same dimensions as the mattress - one reason why they often come in sets. Sometimes the box-spring is mounted directly on casters and a bed frame is not used. According to wikipedia there are three reasons that box springs are used. 1) they provide height - creating a bed that is easier to get into and out of, 2) they reduce wear on the mattress, and 3) they provide a flat, stable surface for the mattress to lie upon.
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