Originally Posted by
Gay
It doesn't matter what size/shape your quilt is, a mitered corner on your border or sashing is folded at 45 degrees, and is not changed because the quilt isn't a square.
With symmetrical border print fabrics, a rectangular mitered border is done differently than a square one. This doesn't involve the angle of the miters, but the side lengths of fabric in a rectangular quilt have to be seamed to create a mirror image if you want all the corners to match.