Both types of sheets can be bought individually or as part of a bed set. A bed set normally consists of either 2 flat sheets and matching pillowcase/s or 1 flat sheet and 1 fitted sheet, again with pillowcase/s to match. It is possible to use flat sheets of a larger size than the actual size of the mattress as the excess can be tucked under the mattress, but when buying fitted sheets it is important to get the right size for the bed – too small and you won't be able to stretch it to fit each each corner, too large and the tautness will be lacking, which means the sheet will move and ruck as you move during your sleep.