Box springs (or box foundations without springs) rest on top of steel support frames. The mattress sits on top of the box foundation.
Steel support frames are not solid. The rails and cross supports have large spaces between them that would allow a flexible mattress to fall through.
A few mattresses are made with built-in foundations that can be used directly with steel springs. The disadvantage is that the mattress cannot be turned over.
Also, the combination of the steel frame, foundation and mattress puts the sleeping surface at the proper height most people are comfortable with.
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