No, it doesn't HAVE to be muslin. Muslin is simply a very inexpensive fabric that is easily available, which is why they recommend it. As other posters have said, you can use anything you have on hand, as long as it doesn't show through your top fabrics. I've purchased sheets on clearance and at IKEA that I've used. Just keep in mind that sheets are usually woven tighter than muslin, and this might be an issue for some quilters.