A bed sheet is a rectangular piece of soft fabric used as bedding to provide a comfortable and hygienic sleeping surface directly on top of a mattress or under other bedding layers. Bed sheets are typically made of materials like cotton, linen, or microfiber and come in two main types: a fitted sheet with elastic corners to hug the mattress, and a flat sheet that is a simple, versatile rectangle placed on top. They serve to protect the mattress from sweat and dust, and are easy to clean and replace.