Backing fabric. If you buy backing of a good quality, it's expensive. But to buy cheap fabrics to use as backing is in my mind a waste of all the hours of work I've spend making the top and will spend quilting it, because the backing will wear so much faster than the top! I want to get as much use out of the back as the top of the quilt, especially for quilts that are meant to be used (i.e. not wall hangings), so expensive fabric it is...