There are two types of sheets called flat bedsheets and fitted bed sheets. Flat bedsheets are not fitted for the mattress. They are like large rectangular pieces of fabric that are laid on top of a mattress. They can be used on their own or with other bedding like blankets and pillows. Fitted bed sheets elastic all around the edges so they fit snugly onto a mattress. They come in standard sizes and can be difficult to fit onto an odd-shaped mattress. Fitted sheets tend to be more secure and stay in place better than regular flat sheets.