Trunle beds (or truckle 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. 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. When not used for sleeping, the lower bed, with mattress removed, can be used as a general storage drawer.