Depends on the size of the quilt in question, what I have available on hand. Sometimes I'll purchase a special wide backing if I can get it on sale, sometimes I use all the orphan blocks and scraps to make up the backing. Did this to a baby quilt a couple years ago and now in the process of making 2 more of the same in different colors (parents' wishes) so will use up the scraps and orphan blocks to add to the yardage I have on hand. Gets rid of the scraps which we all know we have way too many laying around our sewing room.