What's the difference between a box spring cover and a fitted sheet?

Can they be used interchangeably?What's the difference between a box spring cover and a fitted sheet?
Technically, a box spring cover is often like a rectangle with 4 rectangular flaps attached, open on the corners. The large center rectangle lays on the box spring and the flaps drop over the sides. Then the mattress lays on top with a fitted sheet with the stretch corners elasticized and a flat sheet.





And yes, you could possibly use a fitted sheet on the box spring, but since they have no give whatsoever and the corners are hard, you need really stretchy sheets to do it. (Not sure why you'd want to do this though.)