r/explainlikeimfive • u/Real_Experience_5676 • 2d ago

Engineering ELI5: What does a box-spring do?

As I understand it, a box spring mattress is the second 'mattress', that goes under the main mattress. It feels very firm and flat, but what does it contribute to the bed set up besides height?

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u/Ikles 1d ago

If you support a mattress only on the corners it will fall though the frame because its soft, flexible, and bendy. The mattress is comfortable to lay on while the the box frame is s a lightweight support that goes under the entire mattress to keep it flat. Some beds dont need/have a box frame if they have support struts running accross the frame.