Trundle beds (or truckle beds, trumple beds or hurly-beds) are usually considered a pair of beds, one a twin bed, and the other slightly smaller on rollers or casters so that it may be put beneath the upper twin bed for storage. Trundle beds are a space-saving idea and allow for an extra separate bed to be available when necessary, but do not require the second space constantly. The lower bed on some trundles can "pop-up" to be the same height as the twin it is stored beneath, creating a regular-height bed. 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. When not used for sleeping, the lower bed, with mattress removed, may be used as general storage.