A trundle is an extra mattress that is stored underneath your main mattress. You will need, at minimum, enough room to clear the height of the trundle mattress. When not in use, the trundle mattress is stored away and is easily pulled out with the help of a sliding mechanism or casters that enable it to roll out. Some trundles have expanded heights that pop the mattress up to raise it from the ground. Others just slide out and remain a step down from the main mattress.
Consumers usually purchase bunk beds when space is limited in their home. The bunk bed provides a stacked sleeping space for two people. The use of a trundle can add a third bed to the equation and can be pulled out when you have a sleepover. The bunk bed is high enough to allow a trundle to fit beneath the bottom bunk. Bunk beds are traditionally sold as twin over twin but are available in full sizes or a twin mattress over a full-size bottom bunk.
A daybed is a twin-size mattress that is framed on three sides and does not use a box spring. The daybed can serve as a sofa during the day and bed at night. A daybed, because it's not resting on a traditional low-profile bed frame, is high enough off the ground to use a trundle. Most daybeds are supported with a slatted bed base or a link spring design.
A platform bed is ideal for a trundle. They are raised off the ground enough to provide for either storage drawers beneath the frame or the use of a trundle. Some platform bed frames are capable of concealing a trundle in the frame. The trundle slides out like a drawer to reveal the bed. When pushed under, the trundle is hidden behind drawer fronts.