like everyone else said back piece leaves 12 inch gap... u have to have a thick matteress to cover that gap..  wish they would make a piece that covers that gap.. and you WILL need a twinXL mattress.. a twin is too small.  they didn't get me my trundle.. and have "reordered" it hopefully I will get that soon I don't know what size mattress that uses.. prob a twin XL also.