On a well-built and high quality mattress, the side panels will have extra upholstery that would not be added on a more affordable mattress. So, four sides must be contained inside the bed frame and the small join where the zippers are, as otherwise, the mattresses would have to be jammed in and, consequently, be centrally raised. When purchasing a bed frame and a high quality mattress, always ensure that the enclosure on the bed frame is slightly larger than the mattress size.