I've used sheets, also. Then I read that you shouldn't because they are tightly woven and the needle pushes through the threads which weakens them. That's when I started using muslin--which can be very affordable when on sale. Then I got mom's stash, which included sheets for backing. Guess, what? I'm using them. It occurred to me that some of the oldest quilts I have are backed with sheets! Goofy me! Why I didn't make the connection before I just can't figure.