A flat sheet is a rectangular piece of fabric that is placed on top of a fitted sheet and below the comforter or duvet cover. It is also known as a top sheet. Flat sheets come in various sizes, including twin, full, queen, and king, and are typically made of cotton or a cotton blend. They can be used to cover the mattress base or used in the summer and in hotels as a layer on top of your body that covers you while you sleep. Flat sheets can be tucked under the bottom corners of your mattress but may not stay as securely in place because they lack the elasticity of a deep fitted sheet.

The main purpose of a flat sheet is to provide a layer of protection between you and your duvet cover, keeping them clean and fresh for longer. It can act as a barrier for your skin, protecting it from common irritants like dust, bacteria/parasites, and mites. In warmer seasons or climates, a flat sheet can be used as a lightweight covering on its own, providing a comfortable layer without the heaviness of a duvet cover, allowing for a cooler and more breathable sleep experience.

Some people prefer to use a flat sheet without a fitted sheet, while others prefer both fitted and flat sheets. The choice is up to personal preference and comfort. Flat sheets are relatively easy to fold and store, while fitted sheets can be a bit more challenging.