I had some questions about one of the pieced backs I frequently use - an off center 4 patch. I like using this back when I don't have enough of a single fabric to make the backing and when I don't have time to piece multiple fabrics together. In a nutshell - this is how I make the back:
I take 2 pieces of fabric that are long enough to cover the width of my quilt plus 8-10 inches. (my selvages are running horizontally here and the WOF is going to provide the length I need).
I trim one selvage off each fabric and sew the two together.
I make an offset cut so the seam won’t fall in the center of my quilt. (see vertical line)
Flip one side around and sew back them together to create the 4 patch.
This gives me a backing that is about 84 – 88 inches long and to offset my horizontal seam – I just position my quilt near the top of my backing rather than centering it. I also don't worry about having the top and side exactly even although they are in this picture.