Just to add a little to what Rich said. The reason for using a backing is to simulate a single, unviolated ring for a backing. If a stave back is too dificult for the particular bow maker to follow, they could opt to decrown that stave with a plane and glue on a backing, or to chase the ring as best as they can and add a sinew or rawhide backing. Honestly, in my opinion, a wood or bamboo backing is best used on a board bow.