A diagonal seam on the quilt backing will save fabric when the width of the quilt back is wider than the fabric width, but by no more than 50%. For normal 42-44 inch fabric, this will save you fabric on quilts backs no wider than just over 60”. If you have 60” fabric, it will work up about 90”. If you have 90” fabric, it would theoretically work up to a king, but good luck cutting a diagonal on a piece of fabric that big. It is really intended to save fabric when the backing width is a little wider than the fabric, saving the need to buy two full lengths of fabric through the magic of geometry.