Traditionally, fitted sheets are used to cover and protect the mattress.
These are the ones you lie directly on top of and the only thing sometimes used beneath them is a traditional mattress protector.
![White Flat bed sheet by a london brand](https://alondonbrand.com/wp-content/uploads/2018/04/White-Flat-bed-sheet-by-a-london-brand.jpeg)
Traditionally, fitted sheets are used to cover and protect the mattress.
These are the ones you lie directly on top of and the only thing sometimes used beneath them is a traditional mattress protector.