A sheet set typically comes with both top and bottom sheets, also called flat and fitted sheets. While the set usually fits a mattress of the same size designation, the measurements of the fitted sheet -- the one with elastic at the corners -- matter. Some fitted sheets may not fit extra-tall, plush mattresses.
A flat sheet contains no elastic on the edges; instead, it typically goes on top of the fitted sheet, which covers the mattress. The fitted sheet's elastic edges hold it in place on the bed. If it fits well, it stays put, even during a restless night as you try to sleep. Measure the depth of the mattress from top to bottom; then compare the measurement to the dimensions listed on a sheet set to ensure the fitted sheet suits your mattress. The top or flat sheet generally fits the bed well without your having to measure the mattress beforehand, assuming you choose the proper sheet size, such as queen for a queen-sized bed. In a pinch, a flat sheet may be used on a smaller-sized mattress; whereas, the fitted sheet will not fit a smaller-sized mattress snugly.