Foundation vs Box Spring
A foundation is a rigid, non-flexing platform base; a box spring contains actual springs that flex under load. Modern mattresses almost always require the former.
Box springs were designed to pair with traditional innerspring mattresses, adding flex and absorbing shock. Foam, latex and pocketed-coil hybrids need the opposite: a flat, unyielding surface that does not let the mattress deform between support points.
Putting a modern foam or hybrid mattress on a genuine flexing box spring is the single most common cause of premature sagging, and it voids most warranties outright.
For slatted frames the rule of thumb is a gap of no more than 3 inches, plus a centre support leg on any queen or larger. Wider slat gaps let the mattress bulge downward between them and create permanent impressions.
Many products sold today as 'box springs' are actually rigid foundations with a decorative fabric wrap and no springs at all. Check whether the top deck flexes when you press it, rather than trusting the product name.
A true box spring contains coils and was designed to add flex under old innerspring mattresses. Modern foam and hybrid beds require the opposite — a rigid, evenly supported platform — and putting one on an actual box spring will cause sagging and typically voids the warranty.
What most retailers now call a box spring is a foundation: a wooden or steel frame with a solid or closely slatted deck. For slatted platforms, gaps must stay under about 3 inches or the foam will bulge between slats and fail early. Queen and larger sizes also need a center support leg reaching the floor.
What This Means for Shoppers
If your base flexes under hand pressure or has slats more than 3 inches apart, replace it or add a bunkie board before the new mattress arrives — otherwise you will be arguing about a warranty claim in two years.