Muslin comes in many different grades. The less expensive muslins are thinner and work well for foundations. You would purchase the better quality muslin for backings and tops. Better quality means higher thread-count per inch, threads made out of longer-staple cotton, better processing methods used on the thread and fabric, etc. I have purchased top-quality muslin at one quilt shop. It's easy to tell its quality from both the look and the feel of it. Dark-colored threads won't show through it, for example. However, it costs a lot per yard -- the same as other quilt shop fabrics. If you compare this to the muslin purchased for $3/4 dollars per yard at my local JoAnn Fabrics, there is no comparison.