In hotels and other places of accommodation, a trundle bed is a bed that stores itself under another bed, usually on rolling casters. Trundle beds are often found in smaller hotel rooms or in cramped transport accommodations, such as in train suites (couchette) or cruise ship cabins.
For example, a small cabin may have a twin-sized (single) bed, under which a trundle bed is stored. When more sleeping arrangements are required, one can just pull out the trundle bed. A trundle bed may be on a spring, so that it can be raised up to the same height as the bed it’s nested under, or it can just be essentially a mattress on wheels and remain that floor height.