Fitted sheets cover the mattress, held in place by elastic sewn into each of the sheet's four corners. The flat sheet, or top sheet, goes over the fitted sheet and comes between the sleeper and the comforter or blankets on top. Both serve different purposes, and they cannot be interchanged.
Flat sheets look like their name; they are flat, and fold easily and neatly. Fitted sheets, because of their elastic corners, are harder to fold and cannot lie smooth and flat unless pulled over a mattress. Looking for bunches in a folded sheet is an easy way to tell whether you hold a flat or fitted sheet.
Fitted sheets protect the mattress and provide a comfortable surface for the sleeper to lie on. Also called bottom sheets, they fit snugly around the corners of the mattress. A flat sheet lies over the fitted sheet, tucked in on the sides and bottom, under the comforter. Flat sheets help create a welcoming bedspread. They also protect the comforter from getting dirty, so it doesn't have to be washed as often, or they can be used by themselves on warmer nights instead of blankets.
When buying fitted sheets, it is very important to pay attention to the dimensions of your mattress. While it is possible to use flat sheets of the wrong size (using a queen size flat sheet on a double bed, for example), this should not be done with fitted sheets. Fitted sheets must pull snugly over the mattress and tuck in under each edge, so that they cannot be easily pulled off.
Another consideration when buying fitted sheets is the depth, or thickness, of your mattress. Fitted sheets are made to fit up to a certain depth; that number, in inches, will be specified on their packaging. Make sure you know how thick your mattress is before purchasing a fitted sheet, because if the mattress is too thick, the sheet will not be able to round the corners, and will not stay in place.
How to buy
Flat and fitted sheets are often sold as part of a set, which would also include pillowcases. This set makes matching your bedding easy. However, flat and fitted sheets can also be purchased separately. One reason to buy individually is that fitted sheets tend to show wear before flat sheets. Also, some people find flat sheets unnecessary and prefer to use only a blanket on top, while fitted sheets are necessary to prevent sleeping on a naked mattress.
The fabric of your sheets affects their fit and feel, and there are many choices, from Egyptian cotton to Jersey cotton to silk. One factor when choosing the fabric of your sheets is thread count. Thread count refers to the number of threads per square inch woven into the fabric; a higher thread count is considered to be of better quality, because it suggests the sheets will be more durable and comfortable. While you will most likely want the fabric of your flat and fitted sheets to match, higher quality may be more important for your fitted sheet, which will get more wear.