A bedsheet is a large piece of fabric that covers a mattress or lies on top of you while sleeping. Most beds have two sheets: a fitted sheet that stretches around the mattress with elastic corners to hold it in place, and a flat sheet that lies loose on top, between you and your blanket.

Bedsheets keep your mattress clean and make sleeping more comfortable. They're usually made from cotton, linen, or synthetic fabrics, and come in different sizes to match twin, full, queen, or king mattresses. People wash their sheets regularly, just like they wash their clothes.

The word can also appear as two words: bed sheet. You might hear someone say they're “changing the sheets” when they're taking dirty ones off and putting fresh ones on. On a windy day, hanging bedsheets outside to dry can be tricky since they catch the wind like sails and might blow away if they're not properly pinned to the clothesline.