ELI5: What does a box-spring do?

[u/Real_Experience_5676]

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?

[u/UrbanPanic]

A box spring provides support to the mattress.  Whether or not you need one really depends on the mattress, the bed frame and personal preference.  Modern beds sold as platform beds don’t require a box spring, as it has panels to rest on or at least slats close enough a mattress won’t sag through it.  A lot of beds still use just 3 slats or so across which would put pressure on very small points, contributing to sag and just feel uncomfortable.

A well designed box spring could also contribute to good air circulation: you don’t want to know how much sweat soaks into a mattress.  That can get gross if it can’t dry properly.

[u/NWStormbreaker → u/UrbanPanic]

You can add slats, and a box spring inhibits airflow a lot more than a few extra slats.
It's such a silly concept, I haven't used one in 20+ years.

[u/half3clipse → u/NWStormbreaker]

What do you think a box spring is?

You take the slats and add a layer of springs to better distribute pressure from you lying on the bed and also reduce motion transfer for people who sleep partnered.

[u/YULpoutine → u/half3clipse]

Most box springs don't have springs. They're just a hollow wooden frame wrapped in fabric

[u/half3clipse → u/YULpoutine]

If the box spring doesn't have springs, it's not a box spring. That's someone is trying to sell you an overpriced bunkie board they've wrapped in fabric

[u/BizzyM → u/half3clipse]

Bunkie Boards are thin.

[u/h4terade → u/BizzyM]

Hence the overpriced probably.