A trundle is a second bed on castors that lives under the first one. Two people sleep in the footprint of one bed, which is a genuinely good idea and is sold slightly more enthusiastically than the specifications support.
Two things to check before you believe the two-beds claim. The trundle is rated separately from the daybed and it is always lower — often by half. And the trundle needs its own width of clear floor to roll out into, which is floor you have to have.
Castors, locking, and the thing that goes wrong
All the trundles here run on four castors with two locking and two free — the Astoria and the GIANTEX twin daybed both specify it. Lock them when the bed is in use, because an unlocked trundle walks across a hard floor when someone turns over, and it walks away from the daybed, which leaves a gap.
The thing that goes wrong is carpet. Small hard castors on medium-pile carpet do not roll, they plow, and the trundle becomes a two-person job to move. On carpet, a bed with a fixed storage drawer is often better than a trundle, and a plain daybed with a box underneath is better still.
The other failure is the gap. A trundle rolled out is at a different height from the daybed, so you never get one continuous surface — do not buy a trundle expecting to make a king. It is two separate beds at two heights.
The third failure is buying two identical mattresses. A roll-out trundle tray is shallow — the GIANTEX twin daybed’s is 3.5 inches — and the mattress on it has to clear the underside of the daybed as it rolls in, so about six inches is the ceiling there against eight on the frame above. The sizes and the published depth limits are set out in what size mattress for a daybed.
Taken as a whole, though, a trundle is still the most floor-efficient way to sleep two people that exists, because the second bed is stored inside the first one’s footprint. Trundle bed vs sleeper sofa weighs that against the thing a sofa does better for the other 355 days, and the best daybeds for adults is the list if the people sleeping on both levels are grown-ups.
The trundle capacity is the figure that decides who the lower bed is really for, and it is always lower than the daybed above it. The best daybeds for kids treats a 225 or 250 lb trundle as exactly what it is — a child’s bed — while the best daybeds for heavy people lists the two frames that publish enough capacity for an adult on the trundle. If pulling a wheeled bed out at floor level is itself the problem, the best daybeds for seniors covers the drawer-and-no-trundle route.
Three follow-ups to this page, depending on what the trundle did not solve. The British version of this format, and why a pop-up trundle is the one that matters, is in divan guest beds and the US equivalent. The wall-bed alternative, with the floor-space arithmetic, is Murphy bed vs trundle. And if two guests want to share a mattress rather than have one each, the wider frames are in the best full-size daybeds.