Back wings are an aspect of bra design rarely mentioned when talking about styles, fitting, etc. Back wings are the pieces of fabric that go from the bottom of the cup in front to the back. They help secure the bra properly in place.
It is important to know whether the fabric of your backwings stretches and how much. This is also where your bra hooks into place, so the kinds of hooks and how many inches are available in this area is also an important aspect of your overall bra design.