Fitted sheets, sometimes called bottom sheets, are the sheets that cover the mattress on a bed. Fitted sheets often have elastic sewn into each one of the four corners to help it stay in place over the mattress. Fitted sheets, like the other components of bedding, are either sold individually, called open stock, or as part of a complete set. A complete sheet set includes the fitted sheet; the flat, or top sheet; and either one pillow case for twin-sized beds, or two for all other sizes.

Fitted sheets are the one part of a set most people have the most problem with because they don't always stay in place. While most mattresses have a standard width and length, they don't always have a standard thickness. Thicker mattresses pose difficulty when searching for the perfect fitted sheet.

Thicker, pillow top mattresses should be fitted with a wide or deep pocket fitted sheet. Sometimes it is easier to find good fitted sheets sold open stock rather than in a set. Finding one to match a favorite sheet set or complete bedding set may be difficult, but a coordinating fitted sheet that stays on the bed is more favorable to a perfect match when it comes to making the bed.

When selecting flat or fitted sheets, the material, whether Egyptian cotton or polyester blend, is of less significance to quality than thread count. The actual material is mostly a matter of personal preference and comfort more than an indicator of quality, though certain fabrics don't wash as well as others. Polyester blends are more likely to pill than 100% cotton and aren't usually as soft, but they don't wrinkle like cotton does. Blended fabrics that are 50% polyester/50% cotton are preferable to the 80%/20% blend.

When it comes to fabric quality, thread count is the best indicator of the quality of sheets. Thread count refers to the number of threads per square inch in the fabric. A thread count of 200 or higher is desirable and 300 thread count or higher is superior in quality. A thread count less than 200 is unacceptable and though cheaper to buy, costs more in the long run because it will wear out much sooner. Fitted sheets wear out sooner than the flat sheet, so investing in quality, coordinating fitted sheets can extend the life of a complete matching bedding set.