The twin mattress fit perfectly on the trundle bed that came with the daybed we ordered elsewhere. I was concerned that this mattress might be too thick as dimensions are sometimes off on advertised items, but it cleared perfectly to slide under the daybed even with a mattress pad and a thin quilt I am using as a bedspread. Our daybed allowed for about 9.5 inch clearance and this mattress worked. It seems pretty comfortable too. Our grandson will be trying it out soon when his family visits.