I always use material that I used in the quilt. I do however sometimes make the back in a geometric design or large sqs. rectangles and strips. I know a lot use sheets but no matter thread count they are a sheet and the quality of you fabrics in front. I do know a lot use muslin but again issue of quality. If you look at old quilts the backs fall apart even pieces in front that lower quality fab was used. I have a lot of my Grandma's quilts,and u can see where she used good and bad fab. Pieces have just fallen apart. So if u can afford it go for the gold.