Backside sheet is the cloth overlaying the mattress where one typically lies on. The highest sheet is the material that’s laid on prime of the bottom sheet.
The second bed sheet or the highest sheet is where bed covers, similar to duvets, blankets or comforters are bedded.

Sheets are available in forms, the flat and the fitted. Simply in rectangular form is the flat sheet, while a fitted sheet has its 4 corners outfitted with elastic material to maintain it safe on the bed. Mattress clips are sometimes connected to the bed sheet to stop it from slipping off.
When the bottom sheet is flat rather than fitted it’s folded and tucked between the box spring and the mattress when making the bed.

A sheet set is convenient and less expensive to buy in a package slightly than buying them individually. It’s a bundle which features a prime flat sheet, a lower fitted sheet and two pillow cases. They’re typically packed by dimension, twin, king, queen, full and California king.