This bobbin case lets you use thicker threads in your bobbin to do bobbin work on your older B9-hook 9-mm BERNINA sewing machine, including the: 710, 750 and 780 machines. By sewing "upside down", you can produce beautiful designs using decorative threads that would be too thick to go through the eye of a needle. The case provides the proper tension and wider pathway required to handle the threads. The case is a bright red, to easily distinguish it from the regular bobbin case.