A "flat sheet" is a large piece of fabric, typically rectangular, used in bedding. It is designed to cover the mattress and provide a smooth surface for sleeping. Flat sheets are often made from materials like cotton, polyester, or linen, and they come in various sizes to fit different bed dimensions, such as twin, full, queen, and king. Unlike fitted sheets, which have elastic corners to hug the mattress, flat sheets are simply laid on top. They can be used as a top layer or as a lightweight blanket. Many people also use flat sheets for layering or as a protective barrier between the sleeper and the comforter or duvet.