Our fitted sheet (base + zip sheet) is open on the bottom like a traditional fitted sheet, but it extends 3.5” underneath the mattress with sturdy continuous elastic keeping it firmly in place. The base has vertical elastic in all four corners so it flexes to fit a range of mattress thicknesses and hugs the corners snugly. Once the base is on, just zip the zip sheet off and on to change. Please use the link below for a video to how it works.