Trundle beds are usually considered a pair of beds, one slightly smaller than a twin bed that is on rollers or casters so that it may be put beneath the upper twin bed for storage. Trundle beds allow for two separate beds to be available when necessary, but do not require the space constantly. It is a space-saving idea, much like a Murphy bed. The lower bed on some trundle beds can "pop-up" to be the same height as the bed they are stored beneath, creating a larger bed that may be used for guests. Often used as daybeds, trundle beds are less common than bunkbeds for children. Due to storing one bed beneath the other, neither bed usually has a box spring. A double bed A bed is a piece of furniture mainly used for sleeping upon. ... A Murphy Bed is a bed that folds down from a wall. ... A bunkbed is a type of bed in which one bed is stacked on top of another. ...